To add a fillable signature box for someone else to sign in Adobe Acrobat:

1. Open the PDF in Adobe Acrobat Pro.
2. Select All Tools → Prepare a form (if not visible select "View More").
3. Choose the PDF and select Create form.
4. In the form-editing toolbar, select the Signature field icon.
5. Click and drag where you want the signer’s signature box to appear.
6. In the field settings, give it a clear name such as Board Chair Signature, mark it Required if appropriate, and assign it to the correct signer if you are sending it for e-signature.
7. Save the PDF.

To send it out for signatures afterward, use E-Sign → Request e-signatures, add each signer’s email address, then place/assign each signature field before sending. Adobe’s current workflow uses Prepare a form for creating the fields.

If you only need to sign the document yourself—not create a box for others—open the PDF, select E-Sign or Fill & Sign, choose Add Signature, then click where you want it placed. Reader can do this basic self-signing without Acrobat Pro.