Losing Foreign Characters

I have to create a PDF document for our German customers and when I download it from our website it displays it on the browser. However, some of the special characters in the document are not spaced correctly, so I was trying to move them using Adobe Pro. When I open the document in Adobe Pro these characters are missing and just have a square box where the characters once were. How do I prevent this from happening or will I have to go through the whole document adding the missing characters manually?

 From the above example, taken from the PDF opened in the browser, you can see the special characters are not spaced correctly.

 From the above example, taken from Adobe Pro, you can see the special characters are missing.