Editing PDFs in Illustrator is not a good idea. Edits should be done to the original file that created the PDF. When you create a PDF if you do not understand the settings intimately, you can create all sorts of problems. Converting Colro space, compressing images, losing layers, losing editability, etc.
The user should edit the file in the original software that exported to PDF, or they will need to learn how to manage fonts and images. If you still want help trying to fix this in Illustrator, we will need to see the screenshots of the exact fonts you downloaded to help you with using Type >> Find/Replace Fonts. Rather than load the fonts in Adobe would be better if you loaded them for all programs. we will have extra troubleshooting to ensure you load them correctly into Illustrator. Instead use fontbook on mac, or the control panel called Fonts in windows.
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