Make sure you have the latest XP-Pen driver for your tablet and that you are able to open your settings and have cursor control with your pen. 

For Windows:

• Right Click the PDF file and then select Open with WPS office.

• From there, go to Insert -> Graffiti -> Draw curve. You should now be able to write or sign the PDF document.

For Mac:

• Right Click the PDF file, go to Open With and then select WPS Office.

• From there, go to Comment -> Graffiti -> Draw curve,  You should now be able to write or sign the PDF document.

Note: WPS Office does NOT support pen pressure.