adobe reader slow

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

Get the current version of Adobe Acrobat Reader DC (not the Acrobat Pro DC trial) that is built for Big Sur. If it is still slow and you are not opening n-tuple GB PDF, then either upgrade the RAM in your iMac, run fewer concurrent applications, or settle for a slower PDF viewing experience unless you want to use Apple's Preview.

If your internal startup drive is a Fusion drive, consider cloning your current operating system to an external, suitably sized SSD, and booting from that storage. It would be much faster, especially if it were a USB-C cabled SSD connected to one of your TB3 ports (hint: Crucial X8 SSD).

Yes, I know. But you may have some other software installed that's interfering with the reader. We can't do an evaluation without the report.