As a result, the glyphs are not measured and positioned properly. The issue applies to the TrueType and Type1 fonts.

When displaying PDF-Files using PDFViewer, ContentElementsCanvas .RenderAsync uses reflection (DispatcherObjectUtils.ApplyDispatcher) to render Visuals on multiple threads.
However this leads to a memory leak since the dispatchers cant be GC'ed, see screenshot below.




The Screenshot is from the actual application we expirienced this issue with, the attached reproduction example is a boiled down version of what your code does.
If you wanna reproduce this on your own, create an application that uses PdfViewer that switches between many pdf files. The ammount of Dispatchers will grow steadily, probably to a total of the number of threads used by Task.Factory.

Images hosted inside multiple Form XObjects are not rendered.

 Form XObject that contains path with a Matrix position is not rendered.

Some documents cannot be printed with the PdfViewer. In this case no exception is thrown and the result file is 0 bytes. 

When opening a document with a CFF Type1 font an exception is thrown: NullReferenceException: 'Object reference not set to an instance of an object.'

XObejcts with indirect reference to an image are not rendered.

The read-only flag is not respected when importing a document

The images are loaded asynchronously and when scrolling too fast, the loader might fail to load an image if it is bigger.

When the font is not embedded in the document, PdfViewer tries to read it from the OS. However, the Calibri-Font is not properly parsed and a NullReferenceException is thrown internally for its encoding. 

When using SaveAsCommandDescriptor of RadPdfViewer and trying to replace a pdf file that is currently open in adobe reader causes the following exception:

 

System.IO.IOException: The process can not access file "C:\Temp\test.pdf" because it used by another process.
   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access)
   at Microsoft.Win32.SaveFileDialog.OpenFile()
   at Telerik.Windows.Documents.Commands.SaveAsPdfDocumentCommand.Execute(Object parameter)
   at MS.Internal.Commands.CommandHelpers.CriticalExecuteCommandSource(ICommandSource commandSource, Boolean userInitiated)
   at System.Windows.Controls.Primitives.ButtonBase.OnClick()
   at System.Windows.Controls.Button.OnClick()
   at Telerik.Windows.Controls.RadButton.OnClick()
   at System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e)
   [...]

The image inside an XObejct is not shown in the correct position. 

When changing the device culture to RTL (e.g. Hebrew but not only) in a project targeting .NET 5 (or later), some documents are rendered incorrectly.

Represents Highlights PDF content. While the other annotations are drawn on top of the content, the highlight annotation is behind the text it highlights. In other words, it has to be inserted at a specific place in the ContentElementsUILayer. The precise place where the highlight should be placed is described in the PDF file with the BDC, MCID and EMC operators. At the moment, we have implemented a read-only rendering for all annotation types which draws the annotions separately from the page content. In the case of Highlight, the annotation is drawn below the other content. With this implementation, depending on the PDF content, the highlight annotation might be visible or it might not be because in some cases there is geometry content below the highlighted text which hides the highlight annotation appearance. 

A password is asked although Adobe opens the document without one when the document is encrypted with "Encryption of data using the RC4 or AES algorithms with a file encryption key length of 40 bits." - Version 1 (V 1) Revision 2 (R 2).

Similar cases (V1 R3 and V2 R3) are handled in the following item: PdfViewer: A password is asked although Adobe opens the document without one.

When the Interactive Form Fields are in read-only mode, the user should not be able to change the value of the field. The associated Form Field widgets should not interact with the user, e.g. they should not respond to mouse clicks or change their appearance in response to mouse motions.

 Exception when rendering a file with a lot of RenderTargetBitmap objects (Com Exception: MILERR_WIN32ERROR). It seems that we are creating this for tilling patters which are not reused and we ca reconsider our appraoch.  

The text elements in a document are positioned wrongly and are slightly offset from their expected positions.

Currently, there are 10px of margin on each side of the FixedDocumentPresenter element showing the page. This is controlled by the PageMargins property of the PagesLayoutManagerBase class, which is set to a fixed value of 10. To change this, you will need to implement custom PagesLayoutManager and few other page presenter classes from scratch. 

Add a property that allows easily changing of the PageMargins value.

At this moment, you can work this around by setting the PageMargins via reflection.

Currently, RadPdfViewer uses WPF DrawingContext class for drawing page elements to a Canvas.  When PDF page has many content elements (usually geometries) drawing the page may be time-consuming and freezes the UI. 

Instead, we should consider implementing another approach for rendering the content asynchronously which would improve the user experience.