Hello Bella99999,
two (three) possibilities:
1. Format the whole dokument by using a table and protect all cells for changing that contain the fixed parts of your dokument. The "fields" that change their value are unprotected.
2. Define a template with slef filling funktion. All fields that can have changes are defined as placeholders in the dokument. The user will be promted to fill the fields if he opens the template. This makes only sence, if you don't have too many fields to fill.
3. Use the technik of the first suggestion and make this dokument a template. So you prevent overriding your original document when saving after the fill out.
Frank