NovaRecognizing a fillable form
Not every PDF that looks like a form actually has fillable fields — some are just an image or flat layout of a form, with no interactive fields underneath at all. The real test is whether the fields are detected automatically when you open the file; if none show up, the document is a flat form and needs a different approach; typically printing it, or using a tool built for placing text anywhere on a page rather than filling existing fields.
Filling different field types
Text fields take typed input directly. Checkboxes toggle on and off with a click. Dropdowns and radio-style choice fields list whatever options the form's creator originally built in — you can only select from those, not type a custom answer, since the valid choices are fixed by the form itself.
Work through fields in order rather than jumping around, especially on a longer form — it's the easiest way to catch a field you skipped before you get to the end.
Flattening: locking answers vs. staying editable
Flattening turns your filled-in answers into permanent page content, the same as if they'd been printed — the fields can no longer be clicked into or edited afterward. This is the right move once a form is finished and ready to send, since it prevents accidental changes later and displays identically in every PDF viewer.
Leave flattening off if you or someone else might need to revise an answer later, or if the form will be reviewed and adjusted in a few rounds before it's actually final.