In the previous post I was describing the free Zotero scientific manuscript management software. Through a comment via this blog and others, I was pointed out that there are some solutions for access to your PDFs on the go.
1) ZotPad is a 9.99$ (Canadian app store) application that give you access to your collections. You can navigate, search, sort, read, e-mail, get files in and out to annotation software (iAnnotate, GoodReader and others) and sync them back afterward. ZotPad works with the Zotero server and DropBox.
The downside is that you need also to buy a PDF annotation software on your mobile device. Note that the iPad/iPhone version of Papers is also 9.99$ (and you do need the desktop version also at 48$ for students) but you can actually do quite a bit more, including annotate, add notes, tags, …
2) The other option, free(!), is ZotFile2. This piece of software make the link between your Zotero collection and your favorite PDF reader on your iPad or Android device i.e. it will manage the link between Zotero and iAnnotate (for example) designated sync folder. This option might be interesting since it also does a few other things such as extracting annotations in to Zotero note.