What do you recommend doing to a 100-year old document that has been in a drawer for many years?
It has yellowed and is very brittle.

masterframer :
Let's start with what not to do: don't leave it unprotected. If it's in an envelope, caution must be taken in removing it. If it's to be kept in the drawer, I suggest that you place it between two acid-free/rag mats which are then placed within an archival type of folder. If possible make sure the surrounding area is bug-free. No direct strong lighting if it's to be placed out in the open. No extreme heat or cold. And no high humidity. Now some other concerns. If the document has been folded or rolled up all of these years, you may have a problem requiring extreme caution. If it's covered in cellophane tape, you have yet another problem. Depending on just how valuable the item is to you, future care and storage (including framing) can be as simple as leaving it alone or hiring a paper restoration conservator.
Now if it is that you're thinking of framing it, consider your options very carefully. First, remember that once it's framed, it will then be exposed to lighting and conditions of the air around it ( like smoke, humidity,etc.). Second, be sure that you use a professional framer, one who is familiar with conservation practices. You will want to be concerned with matting (museum quality), conservation glass, and conservation quality substrates. Mounting (adhering) it down to a substrate board needs to be discussed: once it's been "glued" down, it's very difficult to remove it from the board. But with proper materials and techniques, mounting may be the best alternative for brittle, crumbling papers. Another alternative would be to make a photocopy of the document, frame the copy, and store away the original. Remember that your document is most likely a one-of-a-kind and therefore, may well be worth the expense necessary to preserve it.

Is it better to use regular glass or non-glare?
Does the non-glare glass help preserve the picture? 

This question is one that the customer usually hasn't given any consideration to prior to being asked by the framer. The concern for preservation involves ultra-violet waves of the light spectrum. Full-blown sunlight is the most damaging in the shortest amount of time (check your drapes on your west windows). Therefore, we use some sort of glazing (i.e. glass or acrylic). We generally think of glass as a protector against dust and such. Well, glass actually helps to cut down the amount of u.v. rays pounding your framed piece. Granted, most rooms aren't filled with natural sunlight, but the stuff does bounce around and reflects off of objects. Also , be very careful of fluorescent lighting (like in offices). With that being said, there really isn't any protective difference between regular/clear and non-glare/non-reflective glass. The concern for choice comes when considering the darkness or brightness of the framed item and the brightness or darkness of the room. That is, dark items tend to reflect more light when behind clear glass, whereas non-glare, which has an etched surface, diffuses the reflection of the light. Some people do not like the look of the slight distortion of their item behind non-glare, especially if matting is involved. If you are concerned about preservation (control of u.v.), then you should consider conservation types of glass. If you are not sure of which to use, clear or non-glare, ask your framer to place a sample of each on your item.

What is the difference between acid-free mats and regular mats?

This is a very good question to always be concerned with no matter what you're framing.
To the majority of our customers, matting means making the picture/object larger overall and/or emphasizing a color present in the picture. However, your framer should be concerned with long term effects that proper matting will incur. That is, professional framers are thinking chemistry: what long term effect will the mat have on the customer's picture/object. There are basically two types of mats: regular and museum/conservation (which also have types within). If your picture/object is only an item of short term display, then regular mats (buffered but not necessarily non-acidic) will do fine. If it is an item of value, one that you wish to preserve for the future, then acid-free/ph neutral mats should be considered. Such mats do cost more, but the long-term effects are priceless. If you have pictures at home that were framed over 10 years ago and these are pictures that are special to you, then I would strongly encourage you to have a professional framer look at them. Older mats may have already created a "burn" line on them and should be replaced.