This is usually an indication for a font problem in your PDF file. Where do you copy the text from, and how to you paste it into your PDF file (comment, form field, static PDF text, ...)? If it's static PDF text that you are trying to create, have you tried to use a different font?
I am coping from a scanned document that is opened in Adobe. When I click the edit button in Adobe DC some of the words garble in the original document so when I copy and paste into the new document it is still gibberish.