Comment by enriquto

You can do this:

    pdf2ps a.pdf    # convert to postscript "a.ps"
    vim a.ps        # edit postscript by hand
    ps2pdf a.ps     # convert back to pdf

Some complex pdf (with embedded javascript, animations, etc) fail to work correctly after this back and forth. Yet for "plain" documents this works alright. You can easily remove watermarks, change some words and numbers, etc. Spacing is harder to modify. Of course you need to know some postscript.