That's what the judge and your lawyers are for. As you say, you can't randomly ask people to read something. It would have been a document that he wrote, or approved, or something -- something already admitted into evidence. The putative purpose would be to ask SBF more questions about the evidence, which is fair.
Remember that SBF never had to take the stand at all -- he could have pleaded the fifth and not testified; which is almost always the right thing to do. But once he decides to take the stand, the prosecutor gets to ask him questions about all the evidence they've collected -- which includes asking him to read things that he wrote or approved of.
Remember that SBF never had to take the stand at all -- he could have pleaded the fifth and not testified; which is almost always the right thing to do. But once he decides to take the stand, the prosecutor gets to ask him questions about all the evidence they've collected -- which includes asking him to read things that he wrote or approved of.