Hi Claudio,
Please let me know where are you tying to view the pdf.
Is it on browser then which browser? Is it using the Adobe Acrobat plugin?
The KB to refer is: Acrobat Help | Display PDF in browser | Acrobat DC, Acrobat Reader DC
If you are not able to open any pdf via Acobat then we need to know the OS details and if any error comes up while opening the pdf.
Is Acrobat set as default application to view pdf files?
Regards,
Rave