In Mathematica 188.8.131.52 under OS 10.15 (Catalina), I have what looks like a reproducible bug, in that it has occurred twice with the same notebook.
In order to create a PDF version, I use the File->Save As option, and select PDF format. The filename is the same as the notebook, but with .pdf instead of .nb. I get the "why the beep" window immediately, warning me that it has created a recovery file since the notebook file has changed, and thanks to this did not lose my work.
The head of both the created pdf file and the notebook file after the "Save as" operation look like this
%%Creator: cairo 1.15.4 (http://cairographics.org)
%%CreationDate: Tue Oct 29 11:44:22 2019
%%DocumentMedia: Letter 612 792 0 () ()
%%Creator: Wolfram Mathematica 184.108.40.206 for Mac OS X x86 (64-bit) (April 8, 2019)
%%BoundingBox: 54 80 506 714
Neither file can be read by Mac preview or Acrobat Reader however.
This looks like a file destroying bug. It's possible that it is somehow specific to my notebook of course, but I am reluctant to start opening up all my notebooks to test this for obvious reasons.
This bug also occurred for me when I attempted to save a notebook as a pdf over an existing .nb file. I haven't found any other mention of this bug and haven't found any way of recovering my work.
On the Mac, I noticed after a recent upgrade that the default name to save as PDF is now the name of the notebook with an nb extension. I'm pretty sure that the default used to have the pdf extension.