Hi Emanuela,
As per the issue description mentioned above, you are facing issue with Acrobat XI on the machine as when trying to view pdf file, you are prompted to sign in to access the trial version of Acrobat XI, is that correct?
As we have checked the account details using the email address you are logged into the forums, you have a perpetual license for Acrobat XI Pro.
Could you try following troubleshooting steps:
1- Download and run cleaner tool on the machine to uninstall Acrobat: Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs
2- Reboot the machine.
3- Download and install Acrobat XI Pro from here: Download Acrobat XI, Acrobat X
You need your Acrobat serial number to complete the installation.
4- Reboot the machine again after providing the licensing the information and check.
Let us know if you need any help.
Shivam