My approach to that is, for letters to the POV character, do not forget that you have a human reading.
Mechanically, I indent the letter like a quote. (Say your normal margins are 1-inch left, 1-inch right: For the letter, I'd use 1.5-inch left, 1.5-inch right.)
But I break that back to normal formatting frequently so my POV can react, stop reading, have other thoughts about what was read, have questions about what was read, have feelings about what was read, imagine scenes about what was read. In one such letter, my POV learns her much older correspondent has a lethal disease and will be dead within a few months, and she stops to cry and grieve over this news, before continuing to read the letter.
Imagine the scene of reading the letter more fully. What it looks like, what any other characters are doing, how the person reading the letter feels. If you don't, you just have an info dump, which is a lot of stuff for the reader to memorize, which they won't, because they only remember the stuff that happened in scenes you helped them imagine.
For letters the POV character finds, I wouldn't give actual text more than half a page, maybe 3/4 but that is pushing it. (a page is 250 words). Instead, time skip it. Meaning the POV character begins reading, finishes reading, and gives her own summary in her own thoughts. As an author, I would limit those to mostly what is actually relevant to the plot.
Again, info dumps are awful because they don't work. They are a lot of information that the author wants the reader to remember, but they won't. They remember the scenes that are emotionally charged, that have action, that show people engaging. Writing a history seems like it should help, but if it is dry it won't.
Show what got read by the reactions of just your POV character, with snippets of what she read that guided her understanding. At times, it is important to show a piece of a letter or something, particularly if it contains a secret or detail your POV will miss, but the reader might catch. Or maybe the POV will realize the true meaning of the letter, later in the book, and the reader that goes back and reads the letter can say "Oh, I get it now". Maybe they got fooled like the POV, or maybe they didn't, so when the reveal comes, they can say "Aha! I knew it!"
Both outcomes are positive.
In textbooks you can infodump, they don't have to be entertaining. Always try to find a way to disguise your infodumping in fiction, reveal the information by having it revealed by other characters, or through the work of a character that you can time skip over, even weeks or months of work, in order to reveal the fruits of that labor, only. Just the emotional take-away from the infodump.
There is a principle stuck in my head from an old Saturday Night Live script, Father Sarducci's Five Minute University. Just what you would actually remember from college classes, five years after you took the class. The actor says, "You take two years worth of Spanish. Five years later, here is what you remember: No Habla Espanol." He then proceeds to go through other classes in the curriculum, American and European history, etc.
Pull a Father Sarducci on your infodump. Sure, your character read it. She can describe the gist of it. But how much will she remember the following week? How would she relate that to a fellow character that hadn't read it? How much of it is truly relevant to her mission? All your READER needs to know is the Father Sarducci version, what she could tell a fellow character, in a conversation, ABOUT what she read, a week later. You don't have to engineer another character for that, this stuff can be in her thoughts, interspersed with action while she thinks and reacts.
As a rule of thumb; follow your POV living her life, and avoid infodumps completely. Write them just to get your own authorial thoughts straight, but don't put them in your book. Even do the Father Sarducci thing on yourself: Write the letter. Put it away for a week. Without reading it again or referring to it, what would you tell a friend, in person, about what it says? Because that level of blurring and generalization should be the take-away of your POV character after reading the letter, without waiting a week.