Ce document postal doit rester dans le dossier jusqu’à demain.

Ce means this or that before a masculine singular noun.

Here, document is masculine singular, so ce document means this document or that document, depending on context.

A quick comparison:

• ce
  • masculine singular noun: ce document
• cet
  • masculine singular noun starting with a vowel or silent h: cet hôtel
• cette
  • feminine singular noun: cette lettre
• ces
  • plural noun: ces documents

In French, many adjectives come after the noun, unlike in English.

So:

• document postal = postal document

The adjective postal describes the noun document, and its normal position is after the noun.

This is very common in French:

• service postal
• adresse postale
• code postal

Postal means related to the mail or post.

So document postal suggests a document connected with postal services, mailing, or post-office procedures.

It is a fairly formal adjective. English speakers may notice it looks very similar to postal in English, and the meaning is close.

Doit comes from the verb devoir, which often means:

• to have to
• must
• to be supposed to

Here, doit is the 3rd person singular present tense form:

• je dois
• tu dois
• il / elle / on doit
• nous devons
• vous devez
• ils / elles doivent

So Ce document postal doit... means This postal document must... or has to...

After a conjugated modal-like verb such as devoir, the next verb usually stays in the infinitive.

So:

• doit rester = must stay / must remain
• peut rester = can stay
• veut rester = wants to stay

This works a lot like English:

• must remain
• has to stay

You conjugate devoir, not rester.

Rester means to stay or to remain, while être means to be.

So doit rester dans le dossier means the document has to stay/remain in the file. It emphasizes that it should continue to be there, not be removed.

If you used être, the meaning would change and would not sound natural in this context.

Dossier usually means file, folder, or case file, depending on context.

In this sentence, dans le dossier most likely means:

• in the file
• in the folder
• in the case record

English speakers should be careful: dossier exists in English too, but in everyday French it is a very normal word, not especially dramatic or secretive.

Dans means in or inside, so dans le dossier means the document is physically or administratively inside the file/folder.

That makes sense here because a document is typically kept in a folder.

A few contrasts:

• dans le dossier = in the folder/file
• sur le dossier = on the file/folder
• au dossier can exist in legal or administrative contexts, but it does not mean exactly the same thing and would not be the most straightforward choice here

So dans le dossier is the most natural wording for to remain in the file.

Jusqu’à means until or up to, and demain means tomorrow.

So jusqu’à demain means:

• until tomorrow

It tells you the time limit: the document must stay in the file up to tomorrow, and presumably something may change after that.

Other examples:

• jusqu’à lundi = until Monday
• jusqu’à midi = until noon
• jusqu’à la fin = until the end

The full form is jusque à, but French does not like this vowel clash, so it contracts to jusqu’à.

This is called elision: the final vowel of one word drops before a word starting with a vowel.

So:

• jusque + à → jusqu’à

This is very common in French:

• le ami → l’ami
• je ai → j’ai
• que il → qu’il

Yes. In many contexts, ce document can mean either this document or that document.

French often does not make the same strong distinction English does between this and that unless needed.

If a speaker wants to be more explicit, they can say:

• ce document-ci = this document
• ce document-là = that document

But in normal usage, just ce document is very common.

A careful pronunciation would be approximately:

Suh doh-kyu-mahn pos-tal dwah res-tay dahn luh doh-syay zhus-kah duh-man

A few helpful points:

• ce sounds roughly like suh
• document has a nasal ending: -ment
• doit sounds like dwah
• rester ends with an ay sound: res-tay
• dans has a nasal vowel
• dossier sounds like doh-syay
• demain sounds like duh-man with a nasal ending

Also, in natural speech, French flows smoothly from word to word, so the whole sentence sounds more connected than English.

It sounds neutral to fairly formal, especially because of words like document postal and dossier.

It would fit well in:

• office language
• administrative instructions
• written notices
• workplace communication

It is not slangy or casual. A more everyday version might depend on the exact situation, but this sentence is perfectly normal and clear in a professional context.