An electronic signature by itself is not the same as a digital signature. An electronic signature is simply a substitute for a handwritten signature, while a digital signature provides additional security measures.
Digital signature
I would recommend that the PDF is generated by AX as normal, then digitally sign it after the higher official has read and approved the document. The signature has to be applied after document generation, because it contains a hash of its content.
The signature could be done by third party software.
Start by reading Adobe's page on the matter.
Electronic signature
AX 2012 has some functionality, start here.
AX 2009, see Dilip's blog, also remember to enable the configuration key.
Microsoft Dynamics AX 2009 does not provide a complete system for the capture and processing of electronic signatures. But it does provide a kernel event and an editable event method. These can be the foundation for an electronic signature subsystem to be developed by partners
The AX 2009 SP1 What's new goes:
Microsoft Dynamics AX 2009 SP1 provides a system for capturing and for processing electronic signatures. The system provides several built-in signatures, such as Bill of Material Approval.
You can enable the built-in signatures by using the Electronic signature requirements form. This form also enables the administrator to define new signatures for tables or fields. Defining new signatures requires careful planning to avoid excessive security which could block proper access to tables.
For more information about using built-in signatures or defining new signatures, see "Electronic signature requirements (form)" in System and Application Setup Help. In the SP1 edition of the Help, see "Generic electronic signatures".