Follow is an example for how to use signature collected in Forms in Workflow.
Use retrieve business process variables to retrieve signature variable
Use update word document to upload word document
In the word document, add a field with text as “«Image:Sig»”(sample Word file attached)
In the upload word document activity, add a new modification and use “Simple Field Merge” type and set the field name to merge to “Image:Sig” and set its value to the signature variable from Forms
In the advanced option of Update Word Document(click the gear icon), select “Create new document” with “Convert to PDF”.
The you can use the result file of the update word document activity to any activity that accept files such as email attachment.
But as it discussed here http://answers.laserfiche.com/questions/64474/Feature-Request-Take-a-Signature-from-Forms-and-Put-Into-PDF#115112, there is legal consideration of putting signature to another document.