Full Use Case: 
I am using Person Accounts as my base object in document action. Since the ‘Email To” field uses only Lead or Contact ID, what should I use so that it merges the Contact from the Person Account?

Under Person Accounts, you will still be able to find a Account Person Contact ID field. When building your merge field code from the custom button, under “Select Field Type” click on “Account” and then under “Insert Field” find your Account Contact ID field, for example: {!Account.Contact_ID__c} then you would paste this field into your document action under the “Email To” field. That way you are still using Person Accounts  but merging the email recipient from the contact ID.