QUICK ANSWERStart with the tool that matches your immediate input.
Open Fill PDF Form Fields first when its stated purpose matches the result you need now. Use Flatten PDF Forms only when it solves a separate next task.
This guide is for students, office teams, administrators, and document-heavy workflows. Start with Fill PDF Form Fields when your immediate task is to use the Fill PDF Form Fields to complete this file task privately in your browser. Move to Flatten PDF Forms only when you also need to use the Flatten PDF Forms to complete this file task privately in your browser.
The goal is not to run two tools automatically. It is to finish the first narrow task, inspect its result, and then decide whether Flatten PDF Forms solves a genuinely different next step.
Both tools sit in PDF Tools, but they handle different inputs or outcomes. Keeping those roles separate reduces repeated work and makes verification easier.