I had the same issue, and after wasting hours of my time trying to fix or investigate this issue on my site https://www.authentictimberwindows.com/. I tested the same links from Semrush in my GSC and Google said there was no problem, so I reported it to Semrush and they.
They said they have a bug which reports PDF crawl issue when JS Rendering is enabled in the audit settings, so the issue is purely a Semrush issue and can be ignored. Very frustrating I know! and shame on semrush.