In addition to the above postings , I'dike to a a few cents worth too .
Double-sided paper items , such as letters & etc. can pose some problems , if it's displaying them (vs only preserving them) is the issue .
As was posted above , light can be very damaging to paper-printed/painted items & often folks will opt for framing them with UV-filtering plastics , rather than just plain old glass .
Flurescent light is a biggie for causing permanent damage to such items , so it pays to not use this type of lighting in display cabinets & in the vicinity of many sensitive items of this sort .
When it comes to adhesives , which are normally required when framing/mounting paper items , I use a fairly recently-invented type :
Product name -ZIG 'Memory System'
Product type - 2 -Way Glue , Xylene-free & acid-free
Manufacturer - KURETAKE CO. LTD , Japan
It's a unique product which can be applied as a 'wet bond' (adhere pieces while the glue is blue & wet) permanently , or as a 'dry bond' (adhere pieces after the glue is clear & dry) temporary/removable style .
This product can be found in art supply/framing supply specialty stores (or can be ordered) , and comes in a small & handy 'pen' size (7g) with an applicator tip , or in larger bottle sizes .
It's a little pricey , but really well worth it .
Geeze ... I sound like a sales rep .....