First, you can save that (uneditable) document as a .doc file?

What you have is not text but a picture. If you had a picture of a car, you could not open the door, get in and drive it. With a picture of text, you cannot edit it.

Go back to the source pdf. You need to get that to be a pdf with text. This is called optical character recognition (OCR). There are a number of programs that will do this; I believe the capacity is built into OneNote.