I don’t know if OmniFocus is really geared towards storing information such as documents long term. I’d suggest you look at Evernote as a free option, and DEVONthink as a paid, and considerably more advanced option.
Both applications can produce links to the documents you store within them, so these links can be pasted into the Notes section of OmniFocus actions. If you have a form that needs to be filled out at a certain time, for example, you can create an OF action with the appropriate due date, paste the link to the form stored in Evernote or DEVONthink, and have convenient access to that document right from OF.
There are other options too for long-term storage, such as Microsoft’s OneNote or Google’s Keep, so you’ll have to look around and see what fits your needs and budget best.