Example:

// Load the document to render
Document doc = new Document(getMyDir() + "Document.docx");

// Create a new class implementing IWarningCallback and assign it to the PdfSaveOptions class
ExRendering.HandleDocumentWarnings callback = new ExRendering.HandleDocumentWarnings();
doc.setWarningCallback(callback);

// We can choose the default font to use in the case of any missing fonts
FontSettings.getDefaultInstance().getSubstitutionSettings().getDefaultFontSubstitution().setDefaultFontName("Arial");

// For testing we will set Aspose.Words to look for fonts only in a folder which doesn't exist. Since Aspose.Words won't
// find any fonts in the specified directory, then during rendering the fonts in the document will be substituted with the default 
// font specified under FontSettings.DefaultFontName. We can pick up on this substitution using our callback
FontSettings.getDefaultInstance().setFontsFolder("", false);

// Pass the save options along with the save path to the save method
doc.save(getArtifactsDir() + "Font.SubstitutionNotification.pdf");

Example:

public void handleBinaryRasterWarnings() throws Exception {
    Document doc = new Document(getMyDir() + "WMF with image.docx");

    MetafileRenderingOptions metafileRenderingOptions = new MetafileRenderingOptions();
    metafileRenderingOptions.setEmulateRasterOperations(false);
    metafileRenderingOptions.setRenderingMode(MetafileRenderingMode.VECTOR_WITH_FALLBACK);

    // If Aspose.Words cannot correctly render some of the metafile records to vector graphics then Aspose.Words renders this metafile to a bitmap
    HandleDocumentWarnings callback = new HandleDocumentWarnings();
    doc.setWarningCallback(callback);

    PdfSaveOptions saveOptions = new PdfSaveOptions();
    saveOptions.setMetafileRenderingOptions(metafileRenderingOptions);

    doc.save(getArtifactsDir() + "PdfSaveOptions.HandleBinaryRasterWarnings.pdf", saveOptions);

    Assert.assertEquals(callback.mWarnings.getCount(), 1);
    Assert.assertTrue(callback.mWarnings.get(0).getDescription().contains("R2_XORPEN"));
}

public static class HandleDocumentWarnings implements IWarningCallback {
    /**
     * Our callback only needs to implement the "Warning" method. This method is called whenever there is a
     * potential issue during document processing. The callback can be set to listen for warnings generated during document
     * load and/or document save.
     */
    public void warning(final WarningInfo info) {
        //For now type of warnings about unsupported metafile records changed from DataLoss/UnexpectedContent to MinorFormattingLoss
        if (info.getWarningType() == WarningType.MINOR_FORMATTING_LOSS) {
            System.out.println("Unsupported operation: " + info.getDescription());
            this.mWarnings.warning(info);
        }
    }

    public WarningInfoCollection mWarnings = new WarningInfoCollection();
}

Example:

@Test
public void enableFontSubstitution() throws Exception {
    Document doc = new Document(getMyDir() + "Missing font.docx");

    // Assign a custom warning callback
    HandleDocumentSubstitutionWarnings substitutionWarningHandler = new HandleDocumentSubstitutionWarnings();
    doc.setWarningCallback(substitutionWarningHandler);

    // Set a default font name and enable font substitution
    FontSettings fontSettings = new FontSettings();
    fontSettings.getSubstitutionSettings().getDefaultFontSubstitution().setDefaultFontName("Arial");
    fontSettings.getSubstitutionSettings().getFontInfoSubstitution().setEnabled(true);

    // When saving the document with the missing font, we should get a warning
    doc.setFontSettings(fontSettings);
    doc.save(getArtifactsDir() + "Font.EnableFontSubstitution.pdf");

    // List all warnings using an enumerator
    Iterator<WarningInfo> warnings = substitutionWarningHandler.FontWarnings.iterator();
    while (warnings.hasNext())
        System.out.println(warnings.next().getDescription());

    // Warnings are stored in this format
    Assert.assertEquals(WarningSource.LAYOUT, substitutionWarningHandler.FontWarnings.get(0).getSource());
    Assert.assertEquals("Font '28 Days Later' has not been found. Using 'Calibri' font instead. Reason: alternative name from document.",
            substitutionWarningHandler.FontWarnings.get(0).getDescription());

    // The warning info collection can also be cleared like this
    substitutionWarningHandler.FontWarnings.clear();

    Assert.assertEquals(substitutionWarningHandler.FontWarnings.getCount(), 0);
}

public static class HandleDocumentSubstitutionWarnings implements IWarningCallback {
    /// <summary>
    /// Our callback only needs to implement the "Warning" method. This method is called whenever there is a
    /// potential issue during document processing. The callback can be set to listen for warnings generated during document
    /// load and/or document save.
    /// </summary>
    public void warning(WarningInfo info) {
        // We are only interested in fonts being substituted
        if (info.getWarningType() == WarningType.FONT_SUBSTITUTION)
            FontWarnings.warning(info);
    }

    public WarningInfoCollection FontWarnings = new WarningInfoCollection();
}