A digital signature identifies the person signing a document as much as a conventional handwritten signature does. Unlike conventional signatures, however, a digital signature also stores information about the person signing the document. Signatures help prevent unwanted changes to a PDF document, which is reason enough why you should be interested in the digital signature capabilities of Acrobat 7.

The first signature in a document is called the author signature. The author who signs a document also has the option of certifying the document. Certifying enables you to attest to the document’s content and specify the changes allowed for the document to remain certified. Changes to the document are detected in the Signatures tab.

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Acrobat supports two kinds of signatures: visible and invisible signatures. A visible signature appears both in the document and the Signatures tab. An invisible signature only appears in the Signatures tab.

A document is best signed only after you have applied the final changes to it, so it doesn’t have to be altered anymore. It’s just the same mind set you have to be in as when signing conventionally: you just don’t sign drafts either.

Visible signature

For this tutorial, I’m going to teach you how to add a visible signature to a document that you own and certify. As the signature image I’m going to use a PDF417 bar code, which is a type of bar code commonly used in passports and official documents.

The first thing you will do is create a digital ID that will be used to sign the document. Digital IDs can be generated by a digital certification service provider. On Windows, such certificates are fully supported by Acrobat, but on Mac OS X 10.3.x you’re pretty much on your own as Keychain does not yet support the exporting of digital certificates. 

This means that on Mac OS X you can only work with digital IDs if they come in an independent .p12 file (which they don’t --the format usually is .p7s). Luckily, though, you can also create self-signed certificates within Acrobat itself.

To create a digital signature, you click on the Sign button. A “Document is Not Certified” dialogue window appears. Click on Continue Signing; you can certify later. A new dialogue will appear, asking you to create a new signature field to sign. You want to sign, so click on Next. Yet another dialogue pops up, asking you to drag an area where you would like the signature to appear.

Obviously, you are going to drag that area with your mouse. The area should be just large enough to hold the signature data. As this is the first time you’re signing a document, you don’t know how large the signature field must be. Well, for starters, it depends on whether you want the signature to contain graphics or not. If you don’t, the field can be kept small. The bigger you draw it, the bigger the signature field’s characters will appear.

If you want to insert an image, which we will be doing here, the field should be about the size of the image you’re using. The reason being that the image is stretched to fill up at least the field’s width, which may not be what you want. Unfortunately, Adobe hasn’t provided a means of measurement for signature fields, so it’s up to your guessing skills to determine how large that field ultimately should be.

The next dialogue window that appears after you have dragged the field area, is the “Apply Digital Signature - Digital ID Selection” window. In this window you can select the digital ID you wish to use to sign the document. The Digital IDs listing will be empty, so you will click the “Add Digital ID” button.