I don't think enough people use them as a primary source, so it may not make sense to give someone credit for a PDF download. I'm sure most people prefer to use the app or mobile site, so PDF downloads are probably not a regular occurrence.
The problem with the PDF is they are set in stone until you generate a new copy and replace the existing copy you have. This isn't always possible as soon as a guide is revised, and the revisions can be major enough it's so different the old PDF is no longer helpful. The main problem is they are static and need to be refreshed so often they probably rarely get downloaded. They do have a use, but the uses that they do well are very limited.
Is it better then nothing if something happens (example: the guide gets retracted and the author doesn't think it's worth saving)? Yes. However, most of the guide here stick unless they are really messy and aren't worth trying to recover so that's rarely going to be the case.