Also, the program is quite lightweight and it is free. I'm not sure if anyone has commented on this program before, but the fact that you can open it from within firefox has made it a dream come true. Then you can simply click 'save' and the file is saved within the zotero library. You can then highlight, annotate, underline, etc. With this program installed you can just double click on a pdf in your library and it will open in the browser. I have used other pdf software (such as bluebeam), but I always had to open them in a separate window. Not only does this free viewer have the ability to open pdfs from within firefox, you can also highlight and annotate the pdfs. This is done by using a free PDF reader/editor called PDF-Xchange viewer. I have benefited tremendously from so many comments in these forums, that I thought I would share a solution that has made managing my work much easier: annotating PDF files opened from Zotero in the web browser.

I was using both Endnote and Bibtex before (still use bibtex, but I export from Zotero).

Download pdf x viewer