Pretty sure that pdf.js from Firefox is safe. At least it runs as sandboxed javascript in the browser. I believe a standalone client may exist as well.
My issue with pdf.js is that it is really bad at copying. every time that I try to copy something from it every word (and sometimes different letters from a word) end up in a different line. I also had issues with rendering, in some (rare) cases I had it show squares instead of the actual content.
Not to mention that it is actually horrifyingly slow compared to most of the viewers that I tried.
