When I split a document, is it better to assign portions of the document or let translators pick what they want to work on?

In most cases, it is much better to assign portions of the document to translators rather than letting them work in a crowdsourcing model. It's not very efficient as translators might jump from one part of the document to another, wasting time and not have a goal to work toward. The best way to do it is ask each translator how many words he/she can complete by the given deadline and assign sections of the document accordingly. If a translator falls behind, it is always possible to change the assignment during the project.

The "free-for-all" model might work when dealing with a very short document with extremely tight deadline for example or for a very large documents where splitting the document might take too much time or where translators might jump in and out of the project. These are not common scenarios.