First, I'd suggest "Find and read". This is because "find" is the first action and it is the action that must take place before the reading. Second, "passive voice sentences" is fine. Indeed it is a bold and confident use of English compound noun phrase construction, but I think "change them into active voice" doesn't quite work. I guess it could be read as a shortened version of "change them into active voice (sentences)". So what I'd suggest is using an article. "... change them into the active voice" or "...into sentences in the active voice". Having made that change it _may_ read better to talk about finding "sentences in the passive voice", but that is really a stylistic rather than grammatical suggestion.