Importing a text document into my iPad so I can modify it

Scan it using the camera.

You can scan the document back into your iPad. Find the Files app, press the "three dot" button in the upper right corner, select scan documents, and follow the on-screen instructions. The result is usually OK, but a real scanner is clearly preferable.

When the document is on your iPad, then you need an app that can edit in PDF files, which I suggest searching for an app you want to use on the App Store, there are many of them, both free and some that cost money .

An example:

https://apps.apple.com/us/app/pdf-expert-pdf-editor-reader/id743974925