make the entry in the Specify File dialog: $PDF
and it should work for you.
If you haven't seen it already, here's a thread on using a variable with save as PDF: Found Sets to PDF with unique file names
other one was tacked onto another person's forum post from last year, but no-one answered
Unfortunately, posts to threads where the initial message is more than 3 months old no longer re-appear in Recent Items when a new message is added to them and thus they are very easy to miss.