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The best way to repair old documents is with 100% Japanese kozo, mitsumata, or gampi fiber paper (sometimes erroneously called "rice paper"), found in art supply stores, and a home-brewed wheat or rice starch paste. Here is a link that explains the procedure: http://www.nedcc.org/free-resources/preservation-leaflets/7.-conservation-procedures/7.3-repairing-paper-artifacts It seems complicated, but if you take it slowly one step at a time, it is manageable and the repair will not harm the document. The Northeast Document Conservation Center (NEDCC) will also answer your phone calls and walk you through the process.
If, after reading it, you decide that it is more than you would like to take on, there are professionals who can do the repair for you. Here is a link where you can find someone: http://www.conservation-us.org/index.cfm?fuseaction=directory.search&specialty=9&pageId=495&parentID=472
They ask a lot of questions at the beginning to make sure the list includes only those who have the technical skills you need for your document.
Of course, one solution is simply to put the document in an archival storage sleeve and place it in an acid-free box without repairing it.
Please let me know if you have questions.
All the best