Check in this order
- Scan. Try to select a line. If the whole page lights up, or nothing does, it is a picture of a page and its words cannot change. Ask for a PDF saved straight from the program that made it. Is my PDF scanned?
- Gibberish on copy. Paste a copied line into a note. Boxes or wrong letters mean we cannot safely change it; it is marked Unsupported.
- Form field. If clicking the text puts a cursor in a box, it is a fillable form. Fill it in a PDF viewer instead.
- Heading or logo that will not select. It was turned into a drawing and cannot be retyped. Ask for a fresh PDF.
- Busy background. A photo or colour fade can show the cover behind new text. It is marked Overlay only. What that looks like.
Open your PDF here and every block is graded before you type: Good match, Limited, Overlay only or Unsupported.
Related: edit the text in a PDF, why fonts change.