Merge Mapping not working in PDF

Hi,

I have installed Adobe Sign  managed package version 21.5.4 in Salesforce.

I have created Agreement template and attached a pdf with merged fields .

When I send the pdf to customer, merge fields are not getting populated.

For Ex. : I am using {{firstname}} and {{lastname}}  in pdf but it is coming same(I should get the data from Salesforce) but when I used document instead of pdf with same merge fields, it worked.

Can you please help me to get it worked with pdf?