Dovada plății și avizul sunt în dosar.

The basic noun is dovadă, meaning proof. In Romanian, the definite article is usually attached to the end of the noun, not written as a separate word.

So:

• dovadă = a proof / proof
• dovada = the proof

That final -a is the definite article for this feminine noun.

plății is the genitive form of plată / plata, and it means of the payment.

Romanian often expresses X of Y by putting Y into the genitive case instead of using a separate word like of.

So:

• dovada plății = the proof of the payment / proof of payment

This is a very common Romanian pattern.

Yes, you may hear both, but they are not always felt exactly the same.

• dovada plății = proof that the payment was made, often referring to a specific payment
• dovadă de plată or dovada de plată = often a more fixed expression for a proof-of-payment document

So in many practical situations they are close, but dovada plății is the more straightforward genitive structure in this sentence.

The base noun is aviz. The ending -ul is the masculine singular definite article.

So:

• aviz = a notice / an approval / a slip depending on context
• avizul = the notice / the approval / the slip

Romanian attaches the article to the noun itself.

Aviz is a context-dependent administrative word. It can mean things like:

• notice
• advice/opinion
• approval
• document/slip

In a sentence about papers in a file, it most likely refers to some kind of official notice or approval document. The exact English translation depends on the kind of paperwork being discussed.

Because the subject is made of two things:

• dovada plății
• avizul

They are joined by și = and, so together they form a plural subject. That is why Romanian uses the plural verb sunt = are.

So:

• avizul este = the notice is
• dovada plății și avizul sunt = the proof of payment and the notice are

Romanian article use does not always match English article use exactly.

Dosar means file, folder, dossier, or case file. In administrative or office-style Romanian, în dosar is a very natural phrase and can often be translated as in the file or in the case file, depending on context.

So even if English uses the, Romanian may still use the bare noun here.

If you want to make it more explicitly definite in Romanian, you can say something like:

• în dosarul clientului = in the client’s file
• în dosarul nr. 5 = in file no. 5

Yes. Romanian word order is fairly flexible.

The version given:

• Dovada plății și avizul sunt în dosar.

is a neutral way to say it.

You could also say:

• În dosar sunt dovada plății și avizul.

That version puts more emphasis on where they are.

So the meaning stays basically the same, but the focus changes slightly.

The special letters here are very important:

• ă as in dovada, plății: a short neutral vowel, similar to the a in sofa
• ț as in plății: pronounced ts
• ș as in și: pronounced sh
• î as in în: a Romanian vowel with no exact English equivalent

A rough pronunciation guide for the whole sentence is:

doh-VA-da pluh-tsee shi a-VEE-zul soont uhn doh-SAR

That is only approximate, but it is a useful starting point.

In this sentence, yes, both are presented as specific, identifiable documents:

• dovada plății = the proof of payment
• avizul = the notice / approval document

Romanian usually marks each noun separately when both are definite.

If you wanted them indefinite, you would normally change the forms, for example:

• o dovadă a plății și un aviz

So the definite forms help show that these are known, specific items in the file.