To create your template:

Options → Picklist set up → Client → HTML email template → click the [+] to add new template → Give it a name → add text and merge fields to email → click attach and select your PDF → save template

 

To send to clients:

Go to client record → document manager → email → click plus to add new email → enter your ID  → Enter subject for email → select new template from drop down list → Click "continue" → Click send in email window top left corner