After seeing the exact error:
Fatal error: Cannot redeclare _PEAR_call_destructors() (previously declared in [xxx]/PEAR.php:773)...
I immediately went searching Google for answers. I found this post: LyftenBloggie Support Article [the link to the external site has since been deleted] which didn't help much, but ultimatly lead me to ask the inevitable question: "What was the last thing I changed?"
The last thing I changed was to add a "latest blogs" module to show on every page of our website. We use LyftenBloggie and although I find it to be a great component to create blogs, it is lacking. Mainly due to the lack of upgrades and support by the developer.
So in short, the LyftenBloggie component was being inserted into every page by accessing the module. Once I disabled this module, the problem vanished. DocMan and any other extension that uses PEAR will have trouble. Although this fixed my issue, the problem still remains... why is there PEAR incompatibility between ANY extension? Shouldn't they all use the same library?