Presumably a corrupted file. Or some kind of incompatibility between the software used to create the file and the software you're using to read it.
Most PDF files have quite a bit of metadata, which can be read using appropriate software tools. This will describe, among other things, the version number of the file and the software used to create it.
(PDF files are ... ahhh ...

designed by committee. You'd think they'd be simple enough. Uh uh. Nope. Zippo. Nada. Nyet.)
Definitely check "the Horse's Mouth" ... Adobe® Reader, which is available for Linux.