Try to leave them in the same condition as you found them. If you can't make a photocopy of the record, extract the important information. Some records cannot be photocopied. They may be too old, or a photocopier may not be available. Don't think you have to write down every word.
Many documents are very long and filled with legal jargon and unimportant phrases.
The best thing to do in these cases is to read the document slowly and thoroughly. Then carefully write down all identifying numbers and important information. Look over every document with a skeptical eye. It's exciting to find an ancestor's old records. You may discover some new and interesting information.