Hi Michal,
Thank you for using Syncfusion products.
Please the find the sample available in the link below to allow signing the PDF document.
http://www.syncfusion.com/downloads/Support/DirectTrac/89901/SecurityPermission1997163196.zipsecurity.Permissions = PdfPermissionsFlags.FillFields;
Use the above mentioned code snippet to allow signing.
Please let us know if you have any question.
Regards,
George