I am having the same problem. When editing a PDF, first all of the letters disappeared, and only square boxes remained. Then my numbers stopped working. Whenever I tried to enter a number all I got was an obscure symbol, that went to the beginning of the line.. I am becoming very frustrated! HELP!
You probably don't have the fonts used in the file on your system. Editing PDF files should only be done as a last measure.
If you have access to the original file used to create the PDF go back to it and perform the edits there, and then generate a new PDF file from it. If you don't have access to that file you can export the PDF to an editable format, like MS Word, and then perform your edits on that file.