PDF amendments change to nonsense when saving a pdf

Hi all,

I'm needing to make some amends to a pdf, but when I save the file, the text I've included changes to nonsense, i.e. changes from "information I've added to the pdf'" to "JJJJGFWBJDUDNE" or some such.

I'm new to using acrobat so not sure if I'm doing something basic incorrectly. Have tried flattening it, trying to embed fonts (can't seem to do this) but nothing seems to work.

Any thoughts on what I'm doing wrong or missing? I'm using Acrobat Pro DC

Thanks!