I've also run into this issue, where Word would suddenly stop saving PDF files using the Save As function.
As I had a large and complex document I needed to compile into PDF on a strict deadline, and as Word didn't tell me which page it became stuck on (a tip mentioned elsewhere in this stack), the following helped me quickly find which page the Word was failing on:
Export the document to PDF in 'blocks'
Note: before you do this, ensure you save a backup (in .docx) somewhere safe first.
Just before pressing Save to transform a Word document into a PDF document using Save As, select Options...
Use the Page range setting to export 'blocks' of the document to see if it works. That will help you narrow down which page Word is failing on.
For example, if you have a 100 page document export pages 1-50 first, then pages 51-100 second. Ensure you try to export the 51-100 'block' even if Word fails on the 1-50 'block' straight up, to exclude any issues in the second half of your document.
If Word fails in the 51-100 'block' but not the 1-50 'block' you know that it's the second half of your document causing the issue.
Keep doing this with increasingly smaller 'blocks' (e.g. 51-70, 71-100 and so on...) until you've narrowed it down to two or three pages.
In my situation I was able to export Page 1, and Pages 7-100 successfully but not the whole document. So I knew the problem existed between Pages 2-6.
Then start removing sections (text, images, tables, headings, hidden formatting), without saving the document to .docx, then try exporting the whole document as PDF again. If it works and Word exports the document successfully, the corruption was in the section you've just deleted.
If Word fails to export to PDF, go back to the document, hit Ctrl + Z to undo the section you've just deleted then repeat by removing another section and exporting the whole document as PDF again.
Rinse and repeat until you can export the document successfully.
Turns out the issue with the document I was working on either was a table, an image, or a section break as the issue seemed to be between two pages.
Also consider Headers and Footers as well, they may become corrupted too. Also found a large image pushed off at the very top of the first page, that wouldn't show when exporting the document to PDF normally, but would show up when exporting the document to PDF with revision comments. Somehow an image got in the revision comments section... shrugs.