This exception seems to be caused because the stream is not decrypted correctly and the stream data is corrupted. For instance when the stream is with FlateDecode filter the exception is InvalidDataException with message "Unknown compression method".

When we render any PDF file with 'x' number of page count and try to enter page 'x+1' in telerik:CurrentPageTextBox and hit enter, it will enable the Previous Page RadButton and if we click Previous Page then it will freeze the application.

System.ArgumentOutOfRangeException: 'pageNo should be greater or equal than 1 and less or equal than 4. (Parameter 'pageNo')'

Workaround: A possible approach could be to attach to the CurrentPageTextBox` KeyDown event in order to restrict the input.

XAML: 

<telerik:CurrentPageTextBox x:Name="PART_CurrentPageNumberTextBox" 
							KeyDown="PART_CurrentPageNumberTextBox_KeyDown"
							Text="{Binding FixedDocumentViewer.CurrentPageNumber, Mode=TwoWay}"/>

private void PART_CurrentPageNumberTextBox_KeyDown(object sender, System.Windows.Input.KeyEventArgs e)
{
	TextBox textBox = sender as TextBox;
	if (textBox != null)
	{
		if (e.Key == System.Windows.Input.Key.Enter)
		{
			int pagesCount = this.pdfViewer.PagesCount;
			int currentPageNumber = this.pdfViewer.CurrentPageNumber;
			if (this.pdfViewer.Document != null && (currentPageNumber < 1 || currentPageNumber > pagesCount))
			{
				RadFixedPage currentPage = this.pdfViewer.CurrentPage;
				int pageNum = this.pdfViewer.Document.Pages.IndexOf(currentPage) + 1;
				this.pdfViewer.CurrentPageNumber = pageNum;
			}
		}
	}
}

When modifying Interactive Forms TextBoxField's value the entered changes are preserved after is clicked out of the TextBoxField's editing area. However the changes are not applied when the click is performed on the save button.

As possible workaround you can raise a MouseLeftButtonDown event on the RadPdfViewer and after this execute the SaveAsCommand. For example:

private void SaveButton_Click(object sender, RoutedEventArgs e)
{
    MouseButtonEventArgs mouseEventArgs = new MouseButtonEventArgs(Mouse.PrimaryDevice, 0, MouseButton.Left)
    {
        RoutedEvent = FrameworkElement.MouseLeftButtonDownEvent
    };
 
    ((FrameworkElement)this.pdfViewer.FixedDocumentPresenter).RaiseEvent(mouseEventArgs);
    this.pdfViewer.CommandDescriptors.SaveAsCommandDescriptor.Command.Execute(null);
}

When extracting text from a composite font the ToUnicode mapping should be used but when this mapping is missing the char codes must be mapped by constructing a secondary map from fonts ordering and registry. 

A possible workaround could be to register a TrueType font using the FontsRepository.RegisterFont() method:

FontsRepository.RegisterFont(new FontFamily(fontName), FontStyles.Normal, FontWeights.Normal, File.ReadAllBytes(pathToFont));

FontsRepository.TryCreateFont(new FontFamily(fontName), FontStyles.Normal, FontWeights.Normal, out FontBase font);

foreach (RadFixedPage page in document.Pages)
{
	foreach (ContentElementBase contentElement in page.Content)
	{
		if (contentElement is TextFragment textFragment)
		{
			if (textFragment.Font.Name == fontName)
			{
				textFragment.Font = font;
			}
		}
	}
}

When importing a PDF document, opening the stream in using statement, and using the default constructor of the PdfDocumentSource class, an exception is thrown:

using (Stream stream = ofd.OpenFile())
{
	this.pdfViewer.DocumentSource = new PdfDocumentSource(stream);
}

using (Stream stream = ofd.OpenFile())
{
	PdfImportSettings importSettings = new PdfImportSettings
	{
		CopyStream = true
	};

	this.pdfViewer.DocumentSource = new PdfDocumentSource(stream, importSettings);
}

This is caused by an incorrect order of applying the TextMatrix and the glyph stroking operation.