Once they're uploaded, your options for editing are limited. There's basic annotation support, and you can create very simple pages and forms from scratch, adding text, art, and data entry fields as needed. You can share files directly through the service, but only for viewing (read-only), not collaboration.
Upgrading to the Premium service removes ads, increases your limit to 100 files (each can be up to 20MB and 200 pages), and pushes the deletion deadline out to 30 days of inactivity. For $20 a month, I can't imagine who would find that to be a worthwhile investment, however, PDFescape's free option should work for users looking for occasional PDF annotation.
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