Typed words on pdf turn to jibberish

I use adobe to mark up a document in red and sometimes I have to add documents to the pdf as proof of some markups. Sometimes (usually at the most inconvenient time) the already saved markups turn to jibberish. Why is it doing this and how do I stop it? "X's" come up all over the mark ups and sometimes just random letters and what I placed on the document is no longer legible.