Files saved in my MAC cannot be opened

[Vasquc]

Some files downloaded from the internet and saved in my mac cannot be opened

[pwillener]

What exactly means "cannot"?  What kind of files are these, and what Adobe product are you using to open them?

[Vasquc]

I have Acrobat Pro xi and the message is :Adobe Reader could not open 'carriee Manual.pdf' because it is either not a supported file type or because the file has been damaged (for example, it was sent as an email attachment and wasn't correctly decoded)."

[pwillener]

Can you open such a file in a text editor, then verify that the first four characters are '%PDF'?

If not, can you find that constant later on in the file?  If so, see http://helpx.adobe.com/acrobat/kb/pdf-error-1015-11001-update.html