Fillable PDFs

Teams’ built-in PDF experience is not a PDF-form editor. Microsoft’s guidance distinguishes Office files, which Teams can edit directly, from non-Office files, which must be edited in their corresponding application. Uploading a fillable PDF to a channel therefore does not make its fields editable in the Teams viewer.
Store the PDF in the channel, then use More options > Open in to open or download it with a PDF application. Complete the fields, save, and upload or replace the copy. If your administrator allows Adobe Acrobat for Teams, it can provide an integrated PDF experience, but available features depend on tenant app policy and licensing.
For a reusable form, keep the original template read-only and have each user create a copy before completing it. This prevents edits or accidental overwriting of the blank master. Test the workflow with the PDF because form types, signatures, and document security restrictions can differ.