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Use Acrobat Pro Prepare Form tool. Click on More on the panel at the right and select "Convert to Adobe Sign Form". Then more input controls field tools including Signature Block will appear at in the toolbar near the top.
Adobe Sign Form Field Tools in the Toolbar
After converting to the Adobe Sign Form, the following input field tools appear in the toolbar: Text Field, Check Box, Radio Button, Dropdown, Signature Field, Signature Block Field, Initial Field, Title Field, Company Field, Name Field, Email Field and Date Field.
Signature Fields
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