PDFSharp doesn't understand pdf files with something after %%EOF.
It's because of code searching for startxref (part of trailer) in last 131 symbols, but it can be before this 131 bytes.
But I'm not sure it's bug - strong specification says that %%EOF must be the last part of file.
More about it : http://stackoverflow.com/questions/11896858/does-the-eof-in-a-pdf-have-to-appear-within-the-last-1024-bytes-of-the-file .
But some C# pdf generators generates pdf with nulls after %%EOF.
I don't know, why they do so..