Acrobat 6 or higher implements a new and more efficient way to compress PDF files: Object Streams. This leads to slightly smaller PDF files than the traditional compression method, which zips large data streams only.
PDFsharp 1.50 beta is able to read many files that contain IRefStreams. Some files still throw exceptions, but we are working on it.
PDFsharp 1.50 does not create Object Streams when saving files. Therefore some files will be slightly larger after they were edited with PDFsharp.
Workaround
If you have files that cannot be processed with PDFsharp, open the PDF document with Acrobat and save it as an Acrobat 5 (PDF 1.4) file.