Le document est accepté, mais il manque la signature.

Est accepté is a passive construction.

• est = is
• accepté = accepted

So Le document est accepté literally means The document is accepted.

French forms the passive the same basic way English does:

• être
  • past participle

Examples:

• Le document est accepté. = The document is accepted.
• La demande est refusée. = The request is refused.

If you used a accepté, that would mean has accepted, and the subject would be the one doing the accepting:

• Le document a accepté... would mean The document accepted..., which does not make sense here.

Because document is masculine singular.

In French, the past participle used with être usually agrees with the subject:

• masculine singular: accepté
• feminine singular: acceptée
• masculine plural: acceptés
• feminine plural: acceptées

So:

• Le document est accepté.
• La demande est acceptée.
• Les documents sont acceptés.
• Les demandes sont acceptées.

The -é / -ée difference shows grammatical gender, not a difference in meaning.

French uses articles much more often than English.

Here, le document and la signature both refer to specific things:

• the document
• the signature

In English, you might sometimes say something shorter like Document accepted, signature missing in a note or checklist. Standard French normally keeps the articles:

• Le document est accepté, mais il manque la signature.

So the articles are not unusual here; they are the normal French way to say it.

Both can exist, but il manque + noun is a very common French way to say that something is missing.

So:

• Il manque la signature. = The signature is missing.

This il does not really mean he here. It works more like an impersonal structure, somewhat similar to English there is in expressions like there is one thing missing.

You may also hear:

• La signature manque.

That is understandable, but Il manque la signature often sounds more natural in administrative or formal contexts.

Here, manquer means to be missing or to be lacking.

So:

• Il manque la signature. = The signature is missing.

But manquer can work in more than one way in French. For example:

• Il manque une page. = One page is missing.
• Ce document manque de clarté. = This document lacks clarity.

So the basic idea is that something needed is absent.

No. In this sentence, il is not referring to a male person.

It is part of an impersonal expression:

• Il manque...
• Il faut...
• Il y a...

In these cases, il does not mean he. It is just a grammatical subject required by French.

So:

• Il manque la signature. does not mean He is missing the signature.
• It means The signature is missing.

Yes, but the structure is different, and this often confuses English speakers.

For feelings, French uses manquer à in a reversed way:

• Tu me manques. = I miss you.
  • literally: You are missing to me.

That is different from:

• Il manque la signature. = The signature is missing.

So the same verb manquer can mean different things depending on the structure:

• something is missing
• someone misses someone

This sentence is the first type.

Because the sentence joins two complete clauses:

• Le document est accepté
• il manque la signature

They are connected by mais = but.

Using a comma before mais is very normal here, just as in English:

• The document is accepted, but the signature is missing.

It helps separate the two ideas clearly.

Yes, it sounds fairly neutral to formal, especially because of the administrative vocabulary:

• document
• accepté
• signature

It would fit well in:

• office communication
• application processing
• official notices
• document review

In casual conversation, someone might say something a bit simpler, such as:

• Le document est bon, mais il manque la signature.
• Tout est bon, sauf la signature.

But the original sentence is perfectly natural and appropriate in formal contexts.

Yes. Some natural alternatives are:

• Le document est accepté, mais la signature manque.
• Le document est accepté, mais il manque une signature.
  This would mean a signature is missing, which is slightly less specific.
• Le document est accepté, mais il n’est pas signé.
  This changes the meaning slightly: the document is accepted, but it is not signed.
• Le document est accepté, mais il manque votre signature.
  This means your signature is missing.

So the original sentence is natural, but French has several ways to express the same general idea depending on how specific you want to be.