It always seems to happen at the worst time. You are at the airport, in a café or waiting at a doctor's office, and someone asks you to send a document "as a PDF, under 2 MB". Your laptop is at home, and the file on your phone is a photo, a Word document or an enormous video. The good news is that modern phones can handle most file tasks if you know a few tricks.

Know what your phone already does

Before installing anything, check the built-in tools:

Document scanning. Both iPhone (in the Notes and Files apps) and many Android phones (in Google Drive or the camera) can scan paper into a cropped PDF.

Print to PDF. On most phones, opening the share or print menu and choosing "Save as PDF" turns a web page, email or document into a PDF.

Photo format settings. iPhones save photos as HEIC by default.…