The filing system proposed and used by Noguchi Yukio is worth a look. To employ the system, you'll need to discard many conventional notions about how to store paper documents. Here's how it works:

You need a set of A4 (letter)-sized envelopes and some way to mark the outside of the envelopes. If you want, you can color-code them with markers.

Take every document and store it in an A4-sized envelope with the flaps cut off, as shown here.

Mark the title and date of the document on the side of the envelope, as shown, and the envelopes are stored vertically on a bookshelf.

Don't attempt to classify documents. The color coding is optional, and only there to help you find documents more quickly.

Add any new document to the left end of the "envelope buffer." Whenever a document is used (i.e., the envelope removed from the shelf), return it to the left end of the bookshelf. The result of this system is that the most recent and frequently used documents move to the left, while documents that are rarely or never used migrate to the right.

Over time, some of the files on the right side of the shelf will be classified as "holy files" which you will retain indefinitely. Remove these from the shelf and store them in boxes. If a "holy file" is in use, it is part of the working file group at the left. Thus, holy files are really dead files which you cannot part with. Get them out of sight into a box.

When you need more space, throw away any documents that you consider "unnecessary."

Read more on Noguchi's system in this article by William Lise, or on Noguchi's website.