Upon loading a PDF, all the form fields will be highlighted with a blue hashing.

Clicking the Show / Hide Form Fields button will toggle highlighting the form fields with the blue hashing.

To fill out a text form field, double click the form field to put it into fillable mode.   You can then start typing and you will be able to fill in any form field with the specified (or substituted) font, size, color, and style as was originally specified by the original form creator.

To check a checkbox form field, select the checkbox form field, double click it to toggle it being checked or unchecked.   You can also right click the selected checkbox form field.   You can select Checked or Unchecked from the drop down for the checkbox form field.

When a form field is selected or in edit mode, pressing TAB will take the user to the next form field.