Documents inserted into ArcMap map documents from other applications via Insert > Object are linked objects. Any updates to the original document automatically update in the map document.
When a document is copied and pasted from another application into a map document, ArcMap pastes a static copy of that picture or text. Any subsequent changes to the original do not update to the copy in the map document.
Insert the document as an object when updates are needed to be made.
Paste the document when the ArcMap document needs to be distributed to others or if updates are not needed.