I had this really strange issue with one of my reports in development. Obviously developing a report is not something you do in one run. Because of this nice bug in Reporting Services I am only able to check the output by generating the PDF through the Reporting Services webservice.
I was getting this funny behavior of PDF files being correct in size and even page numbers but all the pages were empty. When you eventually scrolled through the PDF it would crash saying that the document is invalid.
Apparently the problem is being caused by overwriting the PDF from the .NET application. Deleting the file before creating it seems to solve this issue.
Aha, found the nasty bugger :-(
FileStream fs = new FileStream(path, FileMode.OpenOrCreate);
FileStream fs = new FileStream(path, FileMode.Create);
Developers *sigh* :-D