SF leads' first/last name in PDF not showing

I am testing in Salesforce Sandbox to see if the Adobe Sign works well with our business need.   I have created the merge mapping for a Word document that has {{!FirstName}}, {{!LastName}} etc.   When I send the Word file to the lead, the Word file can show the lead's first/last name properly.   So I do the same for a PDF document (it is a fillable form format) - it also has {{!FirstName}}, {{!LastName}} in the pdf file.  I merge map the same way as I do for the Word document.   However when I send the PDF file to the lead, the PDF doesn't show the First/Last name.  It still shows the {{!FirstName}}, {{!LastName}} in the PDF when it is sent to the lead.   I wonder how I can correct the PDF and make it show the First / Last name.  Thanks everyone..