Modern paper (from the late 1800's to current) is made with quite a bit of acid in it.  This is what causes the paper to brown and become brittle.  In fact, earlier paper doesn't deteriorate nearly as much as modern paper, as it had more rag and a lot less acid.   The acid can also affect items that are put on or in the paper - such as photographs.  However, there is a way to neutralize this acid, and to preserve the paper.  By neutralizing the acid, you can also make it safe for photos and other items!  There is a "paper deacidifier" from Brodart.Com, the Library and Book Supply folks.  It comes in a pump type bottle, and you just spray it on the paper.  It doesn't hurt it in any way, and neutralizes the acid.  It is used on rare and antique books, and will work just fine on the wonderful vintage paper that folks use for scrapbooking.