peter tolmay said:
when recieving certain pdf files by email, they cannot be opened
are you using the newest version of adobe reader?? if you are and it can open them check to see if someone did not you a file compression on them like winzip and then used a password on the zip file. i would also right click on the file after you save it to see if they used a thrid party pdf maker or used a pdf plug in and had word convert the file to pdf.