The tape is pH neutral and doesn't yellow. It also can be removed and applied again during the taping process, which is a big help for those doing this for the first time. Remove old tape if it won't destroy the document finish. It usually falls right off. The mylar top-loader envelopes are fine for storing smaller documents.
Archival companies charge a lot more.
But many documents need bigger storage. From Light Impressions I also bought a pack of large size mylar sheets and a heavier mylar roll in a very large size. To use these, you need double-sided tape. Make sure you purchase Ph neutral tape. Cut two pieces of mylar about one inch larger than the document you have.