PDF-XChange Editor/Viewer Free
This is one of my favorite portable apps for dealing with PDF files.
Using it you can open multiple PDF’s and view them, print them, annotate them within limits on the free verson.
Comments and Annotations
First I’ve heard of this one. Use Acrobat DC at home, Foxit on my laptop. The new Acrobat Tool UI can be a tad annoying.
I’ve been using this one for quite a few years probably at least 8.