Keep everything, paper and electronic.
Letters of Authorizations
STC Certificates
Master Data/Document Lists
Structural & Electrical Drawings
Installation Manuals
Instructions for Continued Airworthiness
Airplane Flight Manual Supplements
Electrical Load Analysis/Supplements
Operating Manual Supplements
Configuration Documents
Parts trace paperwork such as FAA form 8130-3, shipping/pick tickets for equipment other than basic items like tires, brake linings, nuts and bolts.
Invoices for Warranty Claims
Pictures of every page of all logbooks
Equipment Lists and Revisions
All Weight and Balance records
Any burn certifications for interior
AD Compliance Summary
I'm sure I am forgetting something.