http://www.gnupdf.org/Main_Page
http://www.fpdf.org/
http://www.potentialtech.com/cms/node/6
[...]
The second and last links are to PHP classes.
FPDF will help you to generate PDFs using PHP, it will not enable to edit one.
The last link is no longer maintained, but has exactly the same objectives as FPDF.
The third link is the same as the one I provided (please see my reply above for more details)
Please note, gnuPDF is still under heavy development. A version has not been released yet.
Many thanks,
Yousif