Learn French: Le chef doit signer ce dossier. - The boss has to sign this file.

The verb devoir means 'must' or 'to have to'. Just like in English where you say 'must sign' without an extra 'to', in French doit connects directly to the next verb. You never put a word for 'to' between a conjugated form of devoir and the action that follows it.

I thought chef meant a cook. Can it mean 'boss' too?

Yes! While English borrowed this word specifically for a head cook, in French un chef simply means a boss, manager, or leader of any kind of team.

Why does signer end in '-er' here instead of changing to match the subject?

When two verbs appear back-to-back, only the first one changes to match the subject. Since doit is already conjugated to match le chef, the second verb signer (to sign) stays in its basic dictionary form (the infinitive).

Why do we use ce for 'this' in ce dossier?

Ce is a pointing word used for masculine singular nouns. We use it here because dossier is masculine. If you wanted to say 'this' with a feminine word, you would need to use cette instead.