JotNot is an amazing app that can replace a fax machine and a scanner. There is a free version and an enhanced version.
JotNot is really simple to use: you take a photo of a document, you adjust the dimension of the image, you select the format you want such as PDF then you save it or mail it. I use it to save copies of receipts, send PDF images instead of faxing, and to save copies of written notes. It can also send to cloud storage and Evernote.
A second app I recommend if FileApp. I used FileApp rarely before iCloud Drive but now I see that I am going to be using it a lot.
FileApp allows me to read most documents on my iPhone and iPad. The latest version integrates with iCloud Drive so I can store a file, a MS Word document for example, on iCloud Drive from my computer, then using FileApp on my iPhone I can grab that file at anytime and view it.