Identical to this problem: https://acrobat.uservoice.com/forums/590923-acrobat-for-windows-and-mac/suggestions/19583785-combining-pdf-documents-results-in-unprintable-cha

After combining multiple documents, instead of characters showing up as expected when viewing in Acrobat, they show as boxes. When previewing in Edge, Chrome, Nitro, they appear as expected. It has been shown to be version agnostic as well as OS agnostic replicating the issue on Windows and Mac as well as different versions of Acrobat including the most recent patched version.