Just tested this out with a random PDF thrown into one of my album folders.
I ticked the option to show PDF indicator, restarted MB and it showed up fine, went to delete the PDF from that folder since it was just a random test and not related to the album, restarted MB, annnddd... Much of the track info for the first track on the album was blank (Title, Year, Track and Disc numbers, Publisher, Comments). Not the biggest thing as I could easily add back in. But then I also noticed the song's time saying the total time for the whole album and listing the bitrate as unknown. Playing it shows the correct time in the top bar. Had to move the song out of the library, delete it from the library's list, restart MB, and re-add to get it back to working properly, minus losing play count which is fine. The actual metadata for the song wasn't changed at all, just in the MB library. Not sure if the PDF somehow utilizes some aspect of the first track on an album for indexing or so and this broke that.