The full version of Adobe Acrobat is fairly expensive. Most desktop PC users have the free Acrobat Reader installed, but Reader offers few tools to manipulate PDF files. Sejda (currently in beta) is working to bring a fairly full feature set of browser based tools for PDF editing. Features currently offered include encryption, merging, splitting, rotating and combining. Future tools will include cropping, page transitions, text extraction and more. Potentially a very useful and public-spirited tool.