I often receive PDF files which show up as either blank or with blank lines in Preview. Earlier I used to open them in Adobe Acrobat Reader to see the content; however now I've found a better way.
You can open them in Google Chrome -- Chrome uses its in-built PDF reader which shows the content of the PDF file correctly.
I'm not sure why this happens, however it has happened with many PDF files - especially ones which have filled-in forms.
[crarko adds: I don't recall encountering this problem myself, but here is another tool to try if you do.]
Mac OS X Hints