[duskwuff]

This is almost certainly it. I've seen similar issues with copy/paste from poorly constructed PDFs, often ones generated by "print to PDF" features.

[arthur2e5]

Very old LaTeX PDFs tend to have this issue too. Chances are pretty slim for profs to edit PDFs witb Preview, I think…

[duskwuff]

Yep, and in that case it's because those PDFs were often generated through really horrifying pipelines (e.g. TeX to DVI to PS to PDF). Under some workflows, the resulting document wouldn't even contain any characters, as far as PDF was concerned -- it'd just be a bunch of vectors.

[mkl]

Or not even vectors, but lots of little bitmaps. It's really awful.