pdf strange characters on copying or with xpdf

Question

on a linux server I'm using xpdf (whith php) to extract informations from pdf files

On some files all is OK

But on some others the text extracted is very strange !

When I see the pdf on acrobat reader I see normal text

But when extracted I get for example :

de? A8HC8 A
  QFPY 8 A H
        ?f                                   de? A8B=8AN Y?N
                                               QFPXgHBHGe BB

Also when I copy and past from the pdf to notepad or notepad++ on my computer windows 8.1 I see these strange characters

Any help please? regards