Why PDF in the first place. Data storage is cheap to pick up.
We PDF files so that our project managers (non-cad types) have access to the archives, and can do what they need to do without interrupting us with one of their "gotta have it right now!" emergencies...
Whoops, back to reality.
When they do come over and interrupt us, is usually faster to pull up and send the pdf vs finding the drawing file, plotting, etc.
As far as storage goes, I just built a 250GB external USB drive for under $130. Cheap and easy storage option. Better yet, make two and use one for an off site backup. It's not the perfect solution for a ton of reasons, but it gets the job done for most small companies / departments.