Why it happens
Change March to June and the file may have no J in that font, because none was ever on the page. That letter has to come from somewhere else.
What we do
- If the file lets us reuse its font and it has your letters, nothing changes.
- If a letter is missing, usually only that letter is borrowed.
- If the font cannot be reused, the whole changed line is set in a look-alike with the same spacing, such as for Arial, Times New Roman, Calibri or Courier.
What to check
- Read the changed words in the editor before you export.
- Open the downloaded copy and look at those lines again.
- If a word looks off, reword it with letters already nearby; that usually keeps the exact font.
Documents that place every letter on its own can leave spacing slightly different. Those blocks are marked Limited up front.
Related: fix a typo in a PDF, why the text will not change.