Synonyms that are in the dictionary are marked in green. Synonyms that are not in the dictionary are marked in red.
Antonyms that are in the dictionary are marked in green. Antonyms that are not in the dictionary are marked in red.
There are several ways you can do this but the most efficient way is by using dedicated software such as ApowerManager or Tenorshare iCareFone.
Source: https://www.devicemag.com/export-text-messages-from-iphone-to-pdf/