Collaborative editing can be painful and in the end you might end up doing some changes to pdf versions of submitted files. pdfedit comes in handy. I needed some time searching the internet (very high noise level for this topic), while the program kept arguing "document is read-only." How to actually change files? Answer: tools > delinearize, open document, save document under new name, open again. The editor is comfortable enough (although the UI is kind of messy).
P.S.: Inkscape is a very comfortable editor. It allows import and export of many image formats including PDF and EPS. See my blog entry about creating print-quality figures with SVG, where I tell more about inkscape.
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