Well, this is one of those no-brainer posts, but hey - I'll say it anyway, PDF documents are simply not mobile-friendly. They require you to pinch and zoom to really see the content and images in the PDF and thus, are not optimized for mobile devices. So thus, Google does not mark PDF documents in their search results as mobile friendly.
Michael Stebbins shared a screen shot on Twitter of Google marking a PDF on his site as not being mobile-friendly and asked John Mueller of Google why. Here is that screen shot:
John said because PDFs are not mobile friendly. He said he will double check for sure, but he doesn't think Google labels any PDFs as being mobile friendly.
What's a mobile friendly PDF? Would love to learn more!— John ☆.o(≧▽≦)o.☆ (@JohnMu) May 29, 2018
I'm pretty sure we don't flag any PDF as mobile-friendly, but I'll double-check with the team. Mobile-first indexing is independent of whether something is mobile-friendly, so that wouldn't play a role either.— John ☆.o(≧▽≦)o.☆ (@JohnMu) May 29, 2018
Forum discussion at Twitter.