A:AnswerHello and thank you for reaching out about the Wacom Stylus pen is able to work with a Wacom PTK-840. As we are not able to see the photo or description of the pen presented. The KP504E Pro Pen 2 is able is able to work with the PTK-840 Tablet. Hopefully that helps answer your question.
A:AnswerActually, yes it will. This is the new "slim pen" as listed at wacom's site:
https://estore.wacom.com/en-US/wacom-pro-pen-slim-kp301e00dz.html
They specifically mention your tablet part number, which is the same one I have.
A:AnswerYou can view a list of tested devices at www.wacom.com/comp. If your model has Wacom AES or Microsoft Pen Protocol Pen Technology then the Bamboo Ink or Bamboo Ink Plus may be compatible.