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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 field tools including Signature Block will appear in the toolbar near the top.
Adobe Sign Form Field
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Type 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. Choose what you need on the form.
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