Without seeing the file, it's impossible to say what's going on. In general, it's always best if you can go back to the original document that was used to create the PDF file to make the changes. This of course does not help if you don't have access to that file anymore. One thing you can try is to convert the document to e.g. an MS Word document, make your changes in the Word document, and then convert back to PDF. How well this will work depends on how the PDF file was created.
Sorry for the super late reply here. 3 months on the problem is still happening and it happens in every single PDF I edit, whether I create it or not. It seems to be only for Arial Regular as all the other version of Arial seem to work fine.
The only way for me to 'choose' Arial is to click Helvetica (which for some reason doesn't work in PDF) so it appears as Arial. Mind you if I change to Prepare Form, and back to Edit, I have to change the font all over again every time, otherwise it changes to Arial Narrow.
Does this sound like a file issue or a system issue?
Arial works in other programs, seems to only be Acrobat that has the problem.
If you can post a sample file, maybe we can further assist you since we haven't previously seen or heard of this issue.