NOTE: As of later versions of OS X 10.10, and now in 10.11, we cannot reproduce this. It was an inconsistent bug to begin with, and we haven't heard if anyone else has seen it recently. In other words, this may be fixed, but we cannot yet know for certain.
Randomly, the Preview app will fail to render a PDF in a way that VoiceOver can use. In other words, VoiceOver users will sometimes find themselves unable to read PDFs they formerly were able to read with no problem. There is so far no pattern (file size, machine, location, etc) and there are reports that even the same file can be "invisible" or just fine from one day to the next. When a PDF fails to be read, all users will hear is "PDF static text" when they try to read the content of the file.