I use my Android tablet to read long PDF files. How can I add and edit bookmarks within a PDF on Android?
Direct editing of PDF content is something hard to achieve. As far as I know, the only solution for this is Qoppa PDF Notes. However: bookmarks (create & edit) and annotations (also create & edit) are possible.
The best eBook-Reader on Android, also known as Moon+ Reader Pro, supports this not only for PDF (Pro only!) -- but also for EPUB and other formats (Pro & Free). Another popular solution (PDF only) is ezPDF.
For more solutions, you might consult the Playstore, and check with the descriptions. The keywords are "bookmark" and "annotation".
Another Q&A here which might prove interesting to you is: Is there any software for Tablets to edit PDF or PS -files?