What do you mean? This is pretty common here. Nevertheless it should be included in the title if that is what you are getting at.

Yes, putting PDF into the title is considered polite.

On the other hand, if you don't want to open pdfs by accident, use "Always Ask" .. https://support.mozilla.org/en-US/kb/disable-built-pdf-viewe...

What's wrong with links to PDFs? A concern with malware?

I'm not sure about these days, but I do remember being hesitant about opening PDFs 12+ years ago, before I switched to MacOS (Safari has always had very good PDF display ability via OS X), and before all the major browsers had built-in PDF display (PDFs would often be downloaded as a file, which would trigger the opening of some PDF viewer shareware pre-installed on Windows). Terrible UX back then.

Yeah that was true back then. Opening a PDF link on windows 2000 or XP always a pain in the ass. I think those days are mostly over now though.

It used to be considered polite to append "(pdf warning)" to direct links to PDFs. I still like it, even though it's way less of a PITA now than it used to be.

PDFs are often big, which is a concern for those on metered connections.