Garbage Characters appearing

The PDFs showing the white boxes (or scrambled characters) indicate that those PDFs did not have the fonts embedded into the PDF at the time it was created.

So Acrobat used a font on your computer and swapped it for the original missing one in the PDFs. And your font doesn't have the characters the PDF is looking for. Hence, the white box/scrambled characters.

The best solution is to ask the person who created those PDFs to re-export new PDFs, this time selecting the option to embed the fonts into the PDFs. Here are instructions for doing that from MS Word: Embed fonts into PDF with MS Office 

The next best solution is to embed the fonts into the PDF from your computer, but note that there's a good chance you won't have the exact font that was originally used, so the appearance of the PDF's text might change (such as from Helvetica to Arial). Here are the instructions for embedding fonts in Acrobat: Embed fonts into PDF with Adobe Acrobat.