By specification, the last line of the file contains only the end-of-file marker, %%EOF. If the file contains many trailing bytes at its end, a NotSupportedException: 'StartXRef keyword cannot be found.', is thrown.

Workaround: Trim any content after the end-of-file marker (see FixInvalidEndOfFile). 

I have read that there limitations to PDF files being imported, however, the file being imported  was generated with FixedContentEditor, so, should be able to import?

When exporting a document referencing PdfProcessing`s .NET Standard assemblies with PdfComplianceLevel set an exception is thrown.

The common annotation properties are described in table "Entries common to all annotation dictionaries" of the Pdf specification.

Declined: Handling a partial annotation import-export could lead to exporting invalid PDF documents.

According to the PDF Specification: A redaction annotation (PDF 1.7) identifies content that is intended to be removed from the document.

There are two possible options to workaround this functionality:

1. To iterate the document content and replace the matches which meet certain conditions with an empty string:

   foreach (RadFixedPage page in document.Pages)
   {
   	foreach (ContentElementBase elementBase in page.Content)
   	{
   		if (elementBase is TextFragment textFragment)
   		{
   			if (IsValidEmail(textFragment.Text))
   			{
   				textFragment.Text = string.Empty;
   			}
   		}
   	}
   }

   Note: the desired content could be split into several text fragments and if so a custom parser should be created.
   
2. To iterate the document content and check if the TextFragment position is within a specific RectangleGeometry: 
   

   foreach (RadFixedPage page in document.Pages)
   {
   	foreach (ContentElementBase elementBase in page.Content)
   	{
   		if (elementBase is TextFragment textFragment)
   		{
   			if (IsIntersecting(rectangleGeometry, textFragment))
   			{
   				textFragment.Text = string.Empty;
   			}
   		}
   	}
   }

   Note: it should be taken into account that the text fragment could flow out of the rectangle.

When importing a document containing a CIDFont with default width (DW) set as PdfReal (double) an exception is thrown: System.InvalidCastException: 'Unable to cast object of type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Types.PdfReal' to type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Types.PdfInt'.'

44 0 obj
<< /BaseFont /NotoSansMono-Medium /CIDSystemInfo << ... >> /CIDToGIDMap /Identity /DW 600.00000 ... /Subtype /CIDFontType2 /Type /Font >>
endobj

According to the PDF Specification: DW - integer - (Optional) The default width for glyphs in the CIDFont. Default value: 1000.

According to the Pdf Specification: A given object number must not have an entry in more than one subsection within a single section.

The object on line 7 has object number 2, the same as of object on line 5.