All I did was a basic web search (google, bing, yahoo, etc.) for "pdf modifier free" and this came up within the first 5 hits.
PDF-XChange Viewer <-- click link)for Windows XP/Vista/7.
Best part: It's FREE!
I did give it a go and here's what I found:
BOOKMARKS - YES! You can clear the current bookmarks (e.g., usually chapter
markers) and put your own in. What I choose to do was insert my own
"Chapter" marker (i.e., bookmark) at the top of all the other
bookmarks and call it "My Bookmarks". Then, within PDF-XChange Viewer
I can both view the PDF, use the Bookmarks, and add/delete/change my
OCR - WOW! I can scan printed documents in with my scanner (300 DPI minimum) and use PDF-XChange Viewer to OCR the the scanned pages to English, German, Spanish, French or nearly 20+ other languages (which may require the PRO version).
REDACTION - Use the Rectangle tool to "redact" information, e.g., cover text or images with a black rectangle. I used the ability to create my own rctangle "style" of a black solid line and fill rectangle.
Note: To truly (legally) "redact" information you may need to either export the PDF as a TIFF (sadily, each page) then import (or print) those images to a new PDF or physically print the PDF to paper. If you choose to use an app (such as PrimoPDF or CutePDF) to "print" the PDF to a new PDF, consider setting a master password to diable the "copying" of text from within the new PDF. However, do know that the text will actually still be "in" the PDF but have the black rectangle above it, making it difficult to read. Printing a PDF to a new PDF (via PrimoPDF or CutePDF) and securing the document from copying of text MAY NOT meet legal requirements for a true legal "redacting" of information.
So, for free, PDF-XChange Viewer has a good amount of tools to give you power over the PDF.
FYI ~ I'm not realted to Tracker Software, the company who makes PDF-XChange Viewer, in any way. Just a normal person (like y'all) looking for economical tools (free works best!) to get things done.
It's That Simple. Really.