folder of JPEGs created for PDF?

I uploaded some PDFs to a shared folder for a few collaborators to view. A few days and some activity later, I noticed that for some of the PDFs, a folder had been created containing page images of the PDF. What's going on with that?

I can't tell which user's actions caused this, and the other users are non-technical and likely would have no idea that their action caused it. One of the users may have the desktop extension installed, the others almost certainly do not. I have personally viewed the PDFs within Dropbox on the web, and no folder was created due to my action.

Example: suppose I created in the Dropbox folder files abc.pdf, def.pdf, and ghi.pdf. Later I discovered folders (same level as the PDFs) abc, abc (1), and def. Folder abc contains files abc (Page 1).jpg, abc (Page 2).jpg, etc through abc (Page 16).jpg -- the PDF is 16 pages. Folder abc (1) is a duplicate of abc. Folder def is similarly a collection of .jpg files representing the pages of def.pdf. No folder ghi exists, although are least some of the collaborators have viewed ghi.pdf.

It's not a big problem of course -- these aren't huge files. Mostly I'm curious, and want to know if it's safe to remove the mystery folders.