Just heard from Squarespace tech and, while the issue is not resolved, there is a workaround. If you use a Button Block, you can upload a PDF without getting the "status code 500" error.
I created a non-linked, throw away page
Added a Button Block
Uploaded my PDFs
Closed the throw away page and went to my live work page
Linked the uploaded PDFs to the necessary text
Hope this helps. Cheers.