When inserting notes in Preview app, apostrophes become double-byte characters Whenever I type a word with an apostrophe inside a note that I insert in a PDF document in Preview (i.e. "I'm"), the apostrophe automatically becomes a double-byte character. I have no idea why this is happening, nor how to fix it. I'm using Mac OS Mojave 10.14.5

Anybody have an answer on how to fix this?
That's the "smart quotes" feature converting the plain ASCII quote to a fancy (curved) Unicode quote (which is two bytes in UTF-8 encoding). You can turn this off with System Preferences -> Keyboard pane -> Text tab -> deselect "Use smart quotes and dashes". Note that this is a system-wide setting, so it'll affect most word processors as well as Preview.