diff --git a/Pdf4QtLib/sources/pdfdocument.h b/Pdf4QtLib/sources/pdfdocument.h
index cbdb874..6a0d5f8 100644
--- a/Pdf4QtLib/sources/pdfdocument.h
+++ b/Pdf4QtLib/sources/pdfdocument.h
@@ -499,13 +499,14 @@ public:
 
     enum ModificationFlag
     {
-        None            = 0x0000,   ///< No flag
-        Reset           = 0x0001,   ///< Whole document content is changed (for example, new document is being set)
-        PageContents    = 0x0002,   ///< Page contents changed (page graphics, not annotations)
-        Annotation      = 0x0004,   ///< Annotations changed
-        FormField       = 0x0008,   ///< Form field content changed
-        Authorization   = 0x0010,   ///< Authorization has changed (for example, old document is granted user access, but for new, owner access)
-        XFA_Pagination  = 0x0020,   ///< XFA pagination has been performed (this flag can be set only when Reset flag has been set and not any other flag)
+        None                = 0x0000,   ///< No flag
+        Reset               = 0x0001,   ///< Whole document content is changed (for example, new document is being set)
+        PageContents        = 0x0002,   ///< Page contents changed (page graphics, not annotations)
+        Annotation          = 0x0004,   ///< Annotations changed
+        FormField           = 0x0008,   ///< Form field content changed
+        Authorization       = 0x0010,   ///< Authorization has changed (for example, old document is granted user access, but for new, owner access)
+        XFA_Pagination      = 0x0020,   ///< XFA pagination has been performed (this flag can be set only when Reset flag has been set and not any other flag)
+        PreserveUndoRedo    = 0x0040,   ///< Preserve undo/red even when Reset flag is being set
     };
 
     Q_DECLARE_FLAGS(ModificationFlags, ModificationFlag)
diff --git a/Pdf4QtViewer/pdfprogramcontroller.cpp b/Pdf4QtViewer/pdfprogramcontroller.cpp
index ef0ef07..837aac0 100644
--- a/Pdf4QtViewer/pdfprogramcontroller.cpp
+++ b/Pdf4QtViewer/pdfprogramcontroller.cpp
@@ -1057,6 +1057,11 @@ void PDFProgramController::saveDocument(const QString& fileName)
     pdf::PDFOperationResult result = writer.write(fileName, m_pdfDocument.data(), true);
     if (result)
     {
+        if (m_undoRedoManager)
+        {
+            m_undoRedoManager->setIsCurrentSaved(true);
+        }
+
         updateFileInfo(fileName);
         updateTitle();
 
@@ -1091,6 +1096,41 @@ bool PDFProgramController::canClose() const
     return !(m_futureWatcher && m_futureWatcher->isRunning()) || !m_isBusy;
 }
 
+bool PDFProgramController::askForSaveDocumentBeforeClose()
+{
+    if (!m_pdfDocument)
+    {
+        // Nothing to be done
+        return true;
+    }
+
+    if (m_undoRedoManager && !m_undoRedoManager->isCurrentSaved())
+    {
+        QString title = tr("Save Document");
+        QString message = tr("Do you wish to save modified document before it is closed?");
+        switch (QMessageBox::question(m_mainWindow, title, message, QMessageBox::Yes | QMessageBox::No | QMessageBox::Cancel, QMessageBox::Cancel))
+        {
+            case QMessageBox::Yes:
+            {
+                performSave();
+                return m_undoRedoManager->isCurrentSaved();
+            }
+
+            case QMessageBox::No:
+                return true;
+
+            case QMessageBox::Cancel:
+                return false;
+
+            default:
+                Q_ASSERT(false);
+                return true;
+        }
+    }
+
+    return true;
+}
+
 QString PDFProgramController::getOriginalFileName() const
 {
     return m_fileInfo.originalFileName;
@@ -1174,7 +1214,7 @@ void PDFProgramController::onActionOptimizeTriggered()
     if (dialog.exec() == QDialog::Accepted)
     {
         pdf::PDFDocumentPointer pointer(new pdf::PDFDocument(dialog.takeOptimizedDocument()));
-        pdf::PDFModifiedDocument document(qMove(pointer), m_optionalContentActivity, pdf::PDFModifiedDocument::Reset);
+        pdf::PDFModifiedDocument document(qMove(pointer), m_optionalContentActivity, pdf::PDFModifiedDocument::ModificationFlags(pdf::PDFModifiedDocument::Reset | pdf::PDFModifiedDocument::PreserveUndoRedo));
         onDocumentModified(qMove(document));
     }
 }
@@ -1186,7 +1226,7 @@ void PDFProgramController::onActionSanitizeTriggered()
     if (dialog.exec() == QDialog::Accepted)
     {
         pdf::PDFDocumentPointer pointer(new pdf::PDFDocument(dialog.takeSanitizedDocument()));
-        pdf::PDFModifiedDocument document(qMove(pointer), m_optionalContentActivity, pdf::PDFModifiedDocument::Reset);
+        pdf::PDFModifiedDocument document(qMove(pointer), m_optionalContentActivity, pdf::PDFModifiedDocument::ModificationFlags(pdf::PDFModifiedDocument::Reset | pdf::PDFModifiedDocument::PreserveUndoRedo));
         onDocumentModified(qMove(document));
     }
 }
@@ -1626,7 +1666,7 @@ void PDFProgramController::onDocumentReadingFinished()
             m_pdfDocument = qMove(result.document);
             m_signatures = qMove(result.signatures);
             pdf::PDFModifiedDocument document(m_pdfDocument.data(), m_optionalContentActivity);
-            setDocument(document);
+            setDocument(document, true);
 
             if (m_formManager)
             {
@@ -1687,17 +1727,17 @@ void PDFProgramController::onDocumentModified(pdf::PDFModifiedDocument document)
 
     m_pdfDocument = document;
     document.setOptionalContentActivity(m_optionalContentActivity);
-    setDocument(document);
+    setDocument(document, false);
 }
 
 void PDFProgramController::onDocumentUndoRedo(pdf::PDFModifiedDocument document)
 {
     m_pdfDocument = document;
     document.setOptionalContentActivity(m_optionalContentActivity);
-    setDocument(document);
+    setDocument(document, false);
 }
 
-void PDFProgramController::setDocument(pdf::PDFModifiedDocument document)
+void PDFProgramController::setDocument(pdf::PDFModifiedDocument document, bool isCurrentSaved)
 {
     if (document.hasReset())
     {
@@ -1714,7 +1754,7 @@ void PDFProgramController::setDocument(pdf::PDFModifiedDocument document)
             m_optionalContentActivity = new pdf::PDFOptionalContentActivity(document, pdf::OCUsage::View, this);
         }
 
-        if (m_undoRedoManager)
+        if (m_undoRedoManager && !document.hasFlag(pdf::PDFModifiedDocument::PreserveUndoRedo))
         {
             m_undoRedoManager->clear();
         }
@@ -1747,6 +1787,11 @@ void PDFProgramController::setDocument(pdf::PDFModifiedDocument document)
         m_textToSpeech->setDocument(document);
     }
 
+    if (m_undoRedoManager)
+    {
+        m_undoRedoManager->setIsCurrentSaved(isCurrentSaved);
+    }
+
     m_pdfWidget->setDocument(document);
     m_mainWindowInterface->setDocument(document);
     m_CMSManager->setDocument(document);
@@ -1777,7 +1822,7 @@ void PDFProgramController::setDocument(pdf::PDFModifiedDocument document)
 void PDFProgramController::closeDocument()
 {
     m_signatures.clear();
-    setDocument(pdf::PDFModifiedDocument());
+    setDocument(pdf::PDFModifiedDocument(), true);
     m_pdfDocument.reset();
     updateActionsAvailability();
     updateTitle();
@@ -1797,10 +1842,17 @@ void PDFProgramController::updateTitle()
     if (m_pdfDocument)
     {
         QString title = m_pdfDocument->getInfo()->title;
+
         if (title.isEmpty())
         {
             title = m_fileInfo.fileName;
         }
+
+        if (m_undoRedoManager && !m_undoRedoManager->isCurrentSaved())
+        {
+            title += "*";
+        }
+
         m_mainWindow->setWindowTitle(tr("%1 - %2").arg(title, QApplication::applicationDisplayName()));
     }
     else
@@ -2046,7 +2098,10 @@ void PDFProgramController::onActionOpenTriggered()
 
 void PDFProgramController::onActionCloseTriggered()
 {
-    closeDocument();
+    if (askForSaveDocumentBeforeClose())
+    {
+        closeDocument();
+    }
 }
 
 void PDFProgramController::onActionDeveloperCreateInstaller()
diff --git a/Pdf4QtViewer/pdfprogramcontroller.h b/Pdf4QtViewer/pdfprogramcontroller.h
index b99ff3b..a664877 100644
--- a/Pdf4QtViewer/pdfprogramcontroller.h
+++ b/Pdf4QtViewer/pdfprogramcontroller.h
@@ -255,7 +255,7 @@ public:
     Q_DECLARE_FLAGS(Features, Feature)
 
     void openDocument(const QString& fileName);
-    void setDocument(pdf::PDFModifiedDocument document);
+    void setDocument(pdf::PDFModifiedDocument document, bool isCurrentSaved);
     void closeDocument();
 
     pdf::PDFWidget* getPdfWidget() const { return m_pdfWidget; }
@@ -287,6 +287,7 @@ public:
     void setIsBusy(bool isBusy);
 
     bool canClose() const;
+    bool askForSaveDocumentBeforeClose();
 
     virtual QString getOriginalFileName() const override;
     virtual pdf::PDFTextSelection getSelectedText() const override;
diff --git a/Pdf4QtViewer/pdfundoredomanager.cpp b/Pdf4QtViewer/pdfundoredomanager.cpp
index 6c6a51e..faca692 100644
--- a/Pdf4QtViewer/pdfundoredomanager.cpp
+++ b/Pdf4QtViewer/pdfundoredomanager.cpp
@@ -1,4 +1,4 @@
-//    Copyright (C) 2020-2022 Jakub Melka
+//    Copyright (C) 2020-2023 Jakub Melka
 //
 //    This file is part of PDF4QT.
 //
@@ -42,6 +42,7 @@ void PDFUndoRedoManager::doUndo()
 
     UndoRedoItem item = m_undoSteps.back();
     m_undoSteps.pop_back();
+    m_isCurrentSaved = false;
     m_redoSteps.insert(m_redoSteps.begin(), item);
     clampUndoRedoSteps();
 
@@ -59,6 +60,7 @@ void PDFUndoRedoManager::doRedo()
 
     UndoRedoItem item = m_redoSteps.front();
     m_redoSteps.erase(m_redoSteps.begin());
+    m_isCurrentSaved = false;
     m_undoSteps.push_back(item);
     clampUndoRedoSteps();
 
@@ -80,6 +82,7 @@ void PDFUndoRedoManager::createUndo(pdf::PDFModifiedDocument document, pdf::PDFD
 {
     m_undoSteps.emplace_back(oldDocument, document, document.getFlags());
     m_redoSteps.clear();
+    m_isCurrentSaved = false;
     clampUndoRedoSteps();
     Q_EMIT undoRedoStateChanged();
 }
@@ -109,4 +112,14 @@ void PDFUndoRedoManager::clampUndoRedoSteps()
     }
 }
 
+bool PDFUndoRedoManager::isCurrentSaved() const
+{
+    return m_isCurrentSaved;
+}
+
+void PDFUndoRedoManager::setIsCurrentSaved(bool newIsCurrentSaved)
+{
+    m_isCurrentSaved = newIsCurrentSaved;
+}
+
 }   // namespace pdfviewer
diff --git a/Pdf4QtViewer/pdfundoredomanager.h b/Pdf4QtViewer/pdfundoredomanager.h
index 583d09c..45614f5 100644
--- a/Pdf4QtViewer/pdfundoredomanager.h
+++ b/Pdf4QtViewer/pdfundoredomanager.h
@@ -1,4 +1,4 @@
-//    Copyright (C) 2020-2021 Jakub Melka
+//    Copyright (C) 2020-2023 Jakub Melka
 //
 //    This file is part of PDF4QT.
 //
@@ -62,6 +62,12 @@ public:
     /// \param redoLimit Maximum redo steps
     void setMaximumSteps(size_t undoLimit, size_t redoLimit);
 
+    /// Returns true, if document was saved
+    bool isCurrentSaved() const;
+
+    /// Sets flag, if document was saved
+    void setIsCurrentSaved(bool newIsCurrentSaved = true);
+
 signals:
     /// This signals are emitted, when undo/redo action availability has
     /// been changed (for example, user pressed undo/redo action)
@@ -97,6 +103,7 @@ private:
     size_t m_redoLimit = 0;
     std::vector<UndoRedoItem> m_undoSteps;
     std::vector<UndoRedoItem> m_redoSteps;
+    bool m_isCurrentSaved = true;
 };
 
 }   // namespace pdfviewer
diff --git a/Pdf4QtViewer/pdfviewermainwindow.cpp b/Pdf4QtViewer/pdfviewermainwindow.cpp
index 760ebe7..21432d0 100644
--- a/Pdf4QtViewer/pdfviewermainwindow.cpp
+++ b/Pdf4QtViewer/pdfviewermainwindow.cpp
@@ -502,6 +502,13 @@ void PDFViewerMainWindow::closeEvent(QCloseEvent* event)
     }
     else
     {
+        if (!m_programController->askForSaveDocumentBeforeClose())
+        {
+            // User cancelled close operation
+            event->ignore();
+            return;
+        }
+
         if (!m_programController->isFactorySettingsBeingRestored())
         {
             m_programController->writeSettings();
diff --git a/RELEASES.txt b/RELEASES.txt
index 0d8026e..727f433 100644
--- a/RELEASES.txt
+++ b/RELEASES.txt
@@ -1,4 +1,5 @@
 CURRENT:
+ - Issue #43: Save question
  - Issue #40: Sanitization of documents
 
 V: 1.3.2   1.2.2023