Are you looking to from your phone, or are you drafting a new document from several different sources? Google Docs: Online Document & PDF Editor
: You can use "Custom Building Blocks" in Google Docs to save frequently used text sections (like bios or signatures) and insert them into a new document by simply typing "@".
If you need to get messages off your phone and into a single text file:
: For manual assembly and clean formatting, lightweight editors like TextEdit or Notepad++ are reliable for managing and merging plain text or code.