No, really not. PDF is often seen as a magic solution to all sorts of things for which there are much better ways. There are several ways in which you might "edit" a PDF.
1. Edit text already in a PDF. Often possible, often not, always tedious and limited. Indispensible when you need it, but a desperate last resort.
2. Add notes and comments of various kinds. Not relevant.
3. Click to add little blocks of text of various kind. Perfectly fine and easy but not a way to write a letter.
4. Fill in forms. First you spend some time (often considerable time) designing the form. Then you can fill it in quickly and easily (but only where the form fields are). May be ideal for something like a delivery note especially if there is no listing of line items, or never more than a fixed number. Lousy for a letter - the "editor" for typing in is hard work compared to something designed to be used.
I see where you are coming from - you already have a PDF. But this is a classic trap - which has been described as "I'm holding a hammer so every problem looks like a nail". Really, this is something you will be doing often and want to do quickly and efficiently. PDF is neither, though it could do the job.