Missing characters after editing, saving and reopening a PDF in Adobe Acrobat.

Im trying to edit a PDF with Adobe Acrobat but when I insert a page, or edit the text, or save it, it suddenly shows missing characters all over the document.

I have read that this issue could arise because the fonts are not embedded correctly in the original PDF, but this is not my case, since I created the original PDF in Adobe Illustrator, and I also have read that Illustrator embeds the fonts automatically. Also, when I do the Preflight all the fonts appears embedded.

The only solution that I could find that worked, is changing the font for any font, and then switch it back to the font that I want. But this is very time consuming since my PDF has 35 pages and I cannot afford changing all the fonts everytime they mess up.

Do you know any permanent solution for this issue? I have read in this forum that many users are experiencing the same, so Im wondering if Adobe could fix this bug or whatever it is. Or if there's a way to export the pdf from Illustrator, in order to not have missing characters when Im editing it in Adobe Acrobat. 

Any suggestion will be greatly appreciate it.