Some PDF files cannot be opened with PDFsharp and the following exception is thrown:
Cannot handle iref streams. The current implementation of PDFsharp cannot handle this PDF feature introduced with Acrobat 6.
Acrobat 6 or higher implements a new and more efficient way to compress PDF files. This leads to significant smaller PDF files than the traditional compression method, which zips large data streams only.
But this feature is not yet implemented in PDFsharp, but it is very high on the to-do list.
Open the PDF document with Acrobat and save it as an Acrobat 5 (PDF 1.4) file.