play with peepdf (basic)

Tested           : backtrack 5 r2 
Tools             : peepdf, foxit reader 
Title of pdf     : forensics.pdf, linux.pdf and howto.pdf 
Target            : forensics.pdf 

now I learn about computer forensic, and I have a problem with my PDF, my PDF can't open.
This is the picture :

now open peepdf on backtrack tools. And then I type “./peepdf.py -h” to know how to use this tools.

root@archaveliano:/pentest/forensics/peepdf# ./peepdf.py -h 
Usage: ./peepdf.py [options] PDF_file 


Options: 
  -h, --help            show this help message and exit 
  -i, --interactive     Sets console mode. 
  -f, --force-mode      Sets force parsing mode to ignore errors. 
  -l, --loose-mode      Sets loose parsing mode to catch malformed objects. 
  -s SCRIPTFILE, --load-script=SCRIPTFILE 
                        Load the commands stored in the specified file and 
                        execute them. 

after I know, I try to set console mode :
root@archaveliano:/pentest/forensics/peepdf# ./peepdf.py -i /media/BCA8-FC99/forensics.pdf  
Error: Bad PDF header!! () 

ok, now I know that my pdf file have a bad header, so I try to find header pdf. I must have a normal file
pdf , so I take the other file pdf to equal with my forensics.pdf. So I choose linux.pdf and howto.pdf to
equal that header with forensics.pdf

open linux.pdf , howto.pdf and forensics.pdf in hexedit. This is an screenshoot :

look forensics.pdf, that have different with linux.pdf and howto.pdf in header, so I try to edit
forensics.pdf header from AA 50 44 AA  2D AA 2E with 25 50 44 46  2D 31 2E . and save it.

Now I try to open forensics.pdf with foxit reader. And this is the result.

-=CMIIW=-