I downloaded a trial version of Acrobat Pro DC Version 2017.009.20044, for Mac . I able to edit a PDF form by entering regular text.
However, when I tried to insert a tick mark in a text box, it fails. I tried two ways
a) Use the key combination Control-Command-Space bar. This brings up a pop-up menu with special
characters. I choose the character by clicking on it. The cursor advances indicating that a character has
been inserted. But no character is displayed. However, this works for other Mac Applications such as Notes.
b) Go to the Edit Menu in the Menu Bar. Choose 'Special Characters...' Option. Click on the character I want. Nothing happens.
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