What makes it stand out from the other PDF annotators in this list is that it allows you to share your marked-up PDF file with a shortened link, and even with embed codes! Once you share it, you can choose to let your viewers add comments to your document. The other unique feature is that you can collaborate with others so you can all view and annotate the file together, even if you’re in remote locations. You can then see all of the changes made to the document in a sidebar. Crocodoc allows you to download the original file as well as the annotated version, which you can view in Sumatra, Evince or your favorite PDF reader, with annotations and all.

And the greatest drawback to this method: it’s one-way. If you make any changes to a reference within Mendeley, those changes will not be reflected in Zotero. This drawback is most apparent to me when I read PDFs. Let’s say I have a reference with an attached PDF in Zotero and I want to read it on my iPad (the Mendeley App has a native PDF reader, but you can also open PDFs in external apps, like iAnnotate). If I’m using the Zotero→Mendeley Desktop→Mendeley iPad app method, and I open that PDF on my iPad, any annotations I make on the PDF will not show up in Zotero.