You can't.
Thats the "magic" of pdf; you can distribute you document and be sure it won't get modified. For this, you must have the source of the document.
You may change a bit of text but its really painful. You surely can't change pictures.
Maybe you could try to copy everything, paste it into a word document, and convert it to pdf after with Acrobat Pro (not the free reader)
Adobe Acrobat 6.0 Professional
US$449.00
http://www.adobe.com/store/products/master.jhtml;jsessionid=I4YYTYDGTZ1NLQFI0ILB2SOAVDJBIIV1?id=catAcrobatPro
or
Adobe Acrobat 6.0 Professional upgrade
for licensed users of Acrobat 4.x, 5.x, or 6.0 Standard
US$149.00
http://www.adobe.com/store/products/master.jhtml;jsessionid=I4YYTYDGTZ1NLQFI0ILB2SOAVDJBIIV1?id=catAcrobatPro
Or you could try OpenOffice.org v1.1 RC2 @
http://www.openoffice.org wich may do pdf document for free
good luck!
big
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Edited:
Oups, RC3 is already out