To put form fields into an existing PDF: 

Select ‘Forms’ from the menu bar 

Select ‘Add or Edit Fields’ from the menu. (Choose ‘No’ when asked if the programme should detect the fields for you.) 

Select ‘Add New Field’ 

Select ‘Text Field’ from the list shown 

Draw the text box where you want it to go 

Give it a unique name (e.g. Field1) 

Click the ‘Show All Properties’ link 

Go to the ‘Options’ tab and enter text with a tag exactly as you would in Word (e.g. <>) in the ‘Default Value’ box. 

You can change the alignment and any other properties within the dialogue box if required.

The text in the default value is what gets processed for replacement tags. 

Add other fields as required and save the PDF. 

Upload it to Docmail and the process is the same as for a Word file - Docmail will extract the form fields from the document as merge tags.