Viviane:
FYI, when I downloaded this, three different virus and malware scanners picked up adware embedded.
Documents with text only are harmless. Some documents can be enriched with active content, like scripts. These scripts are run by the rendering software, like your PDF reader. If the script is malicious and your software vulnerable to the code written in script and executed, then your system might be compromised.
Solutions:
Disable scripting in PDF software.
Use Foxit Reader rather than Acrobat Reader.
Try Foxit. It's 1.5MB versus 150MB of Acrobat, does not install 700 useless registry keys, 10 times faster, 10 times smoother, no vulnerabilities, no need to reboot to upgrade.