Manage Document Library Across Devices

Hi – I'm a graduate student in a field that involves quite a bit of reading and writing. I'm digitizing my hardcopy book library into searchable PDFs so that I'll be able to access my entire library on my iPad/iMac/iPhone. I've only just begun the process. I'm looking for the best cloud/cross device ecosystem for accessing and annotating PDFs (preferably so I don't have to manually redownload them every time a change is made), whether they are complete books or academic papers, and writing notes or paper drafts (I complete my drafts on my iMac).

For some time now I've been using Dropbox because it stores files remotely as well as locally both on my iMac and MacBook and seamlessly syncs between the two; I can work on a word document or annotate a PDF on my iMac, save it, and open it on my MacBook without having to redownload it (or rather Dropbox downloads the newest files automatically replacing the old). My problem is that I don't know how to get my iPhone or iPad in on this. I've been using Documents and PDF Expert but the big drawback is that whenever I access something from Dropbox I have to manually download it onto my iPad or iPhone and then the changes I make to the file I have to email to myself and replace the document in my Dropbox smart folder; on my iMac or MacBook I just open the file (whether it's a pdf or word document) in the Dropbox smart folder and when I'm done I simply save my changes and Dropbox automatically updates the folder system/file on the other computer accordingly — quite convenient! But I want to rebuild my 'ecosystem' around my iPad and iMac rather than around my iMac and MacBook so that I can seamlessly annotate or highlight PDFs, and make changes to word documents or notes, without having to manually update each change on the other device. But at the same time I do want to the files stored on the device if possible (with the cloud mediating between the two like with Dropbox). Is this possible with iCloud? I've been considering switching over to iCloud completely so I don't find myself divided between the two cloud services. I like the way iBooks, for example, syncs between my iMac and my iPad so that I have everything on my iPad (I.e. I don't have to be on wifi to access my books) – only the iPad annotating features for PDFs in iBooks is pretty much nonexistent! Is what I'm trying to do even possible?

In short, what's the best way to maintain a fluid document workflow between an iMac and an iPad, one that involves an entire library of Word documents and PDFs that are continually being revised and annotated? Would it be best to drop Dropbox and switch to the iCloud?