PDF preview not available
Instead of opening PDF (e.g. https://trac.torproject.org/projects/tor/attachment/ticket/10591/graph.pdf), Trac says
HTML preview not available, since no preview renderer could handle it. Try downloading the file instead.
If it's too hard to add a renderer, then it might be enough to add a link to PDF (not to download PDF, the reason is #18220 (moved)).