For Foxit PhantomPDF version 10 and later
Since Foxit PhantomPDF v10,both of form fields and objects(including text, image, path, and shading objects) could intelligent detect grid for fields and objects alignment when you create or move a form field or object. With the "Snap to Grid" feature turned on,you can align the edge(s) of form field/object to the neareset grid line.
Note:the snap to grid can not work for annotations/comments alignment.
For Foxit PhantomPDF versions prior to V10
In Foxit PhantomPDF v9 or older,the snap to grid works only for form fields alignment when adding or moving form fields.When you create or move a form object with any of tool under the Form tab in Foxit PhantomPDF, please check Grid function and Snap to Grid under the View tab, then add or move form fields, the size of the field will be snapped to grid.
If you need to change the grid spacing,please go to File tab>Preferences>Layout Grid to change the value of Width between lines and Height between lines there ,click on OK to save the setting.