Contrary to common belief, the translation process is not linear. Sending documents in dribs and drabs and expecting them to be returned individually as and when they’re ready could be detrimental to the quality of the work.
I once translated a long list of independent, one- or two-sentence paragraphs for the purpose of a presentation leaflet. There was no time to ask for more background on each item, so I did my best to research what was less clear, which still took a fair amount of time. A couple of days after I delivered the work, the agency sent me the bigger part of the project (out of the blue, as far as I was concerned): a detailed event programme from which, it turned out, the previous snippets of texts had been extracted. Now that I had the full context, I realised that some of my initial translations were actually incorrect. I had to re-translate the sentences in this new text even though the end-client had requested that what had already been done be not translated again. This back-to-front organisation – largely due to the client seeing translation as a mere word-for-word task – saved neither time nor money and the leaflets had to be redone. Had the programme been sent to me first, the initial task of translating that series of sentences would have been redundant.
Words or even sentences don’t work in isolation; they are part of a whole. A translator typically goes back and forth, changing their choices of words as they progress and better understand the text. What seems to mean one thing on the first page may prove to mean something else when they reach the third. Or there may be a similar term cropping up later, requiring a more specific or slightly different translation of the first term to better distinguish between the two concepts. In short, the translator needs to have a broad perspective on the text to ensure accuracy and cohesion.
For a stress- and chaos-free process as well as to ensure coherence, it is best to send all your documents together so your translator can check that all parts of the work make sense as a whole.