I usually take a photo of documents that there's a small chance I would need in the future. This way, they take up a very small amount of space on my phone but in case I need something again the photo will usually be sufficient and I can search by date to find it.
I do something similar: put in a pile and about once a month I scan the pile (I bought a printer with a paper-feed just for this), upload it to cloud storage, and date it.