IN terms of into vs in: "Into," is used to signify movement - a change from a situation where you were not inside something to one where you are: Examples: 1) I am in the park. 2) I am just walking into the park now. You should use "onto," to also signify a change in status. In terms of a document, an English speaker would probably say: 1) "I am adding this into the document," if they were adding/ inserting information within the text that already existed in a document, but 2) "I am adding this onto the document," if they were adding extra information - such as an appendix, at the end of a document. I do not think that a native speaker would say " I am adding this in the document," because you need the suffix "to," to indicate that a change is taking place. "Onto," also has the same sort of meaning as "on top of." "I placed the book onto the table." = "I placed the book on top of the table."

I'll have a go at answering this for you. I am not an English language teacher/Instructor/tutor however my understanding is as follows: Into means to put something inside something else. Onto means to place something on top of something else or to add it to something else. Example. Put something into a document To put a letter into an envelope Put that rubbish (garbage/trash if American) in the bin. From my personal experience I use into and in interchangeably, but not always "Put that onto my account" or "add that onto / to my bill" in - https://en.oxforddictionaries.com/definition/in into - https://en.oxforddictionaries.com/definition/into onto - https://en.oxforddictionaries.com/definition/onto https://en.oxforddictionaries.com/definition/on_to