You can use SignRequest on all devices (phones, tables, laptops or desktops).
On devices with Internet Explorer 8 or earlier the service might not work really smooth, but even Microsoft doesn't support these anymore.
If you believe your signature doesn't look good enough we recommend using a mobile device to create your signature. If you log in to a mobile device the signature will be accessible on your other devices as well. (You might have to refresh the page.)