Hi jackouille, Immunet does automatically delete the history files on occasion to keep these files from becoming too large and hogging up HDD space. When this happens the UI may show that a scan and/or updates have never been performed. To correct this just manually update and/or run a quick flash scan.
If that doesn't work then there is another possible cause for this. Your .db history files have become corrupted for some reason. If this occurs usually a simple uninstall & reinstall will correct this issue.
Acrobat Reader can be vulnerable to certain types of exploits by infected PDF files so that could be a legitimate quarantine response. Older versions of Acrobat Reader have memory leak issues that actually makes them even more vulnerable to exploits. What was the detection name?
Then again, if you are "absolutely sure" it is a False Positive you can restore the program with Immunet by clicking on Quarantine, find the file(s) associated with Acrobat Reader, click on the file and choose the Restore option. This will automatically place the file(s) in Immunet's Exclusion list. Files in the Exclusion list will no longer be scanned so be careful with restoring quarantined files.
Personally I use a Firefox add-on that can unpack & display PDF files from my browser without the need to download the file and use a dedicated third-party PDF reader like Acrobat Reader.
Best wishes, Ritchie...