Many photographers utilize PDFs in a variety of ways. The number of possible uses is only limited by the creativity of the photographer and photographers are known to go to extremes to take a creative picture.
To start, a professional photographer deals with contracts all the time. Each client has expectations of what they will get from their wedding or other event. All the details are specified in the contract so there will be no misunderstandings.
A secured PDF makes sure none of the terms or conditions are changed. A laptop, or tablet with the boilerplate PDF forms can even attach addendums to the agreement and be marked with the proper signatures for approval. Many times, clients request additional services once the photographer is on site and a signed addendum will eliminate problems when the final bill is delivered.
Speaking of billing, a secured PDF detailing the bill can be emailed right to the client when you are finished with the event.
A professional photographer shooting for stock photos can carry a tablet or smartphone that has a photo release form. When a person in the picture has given verbal permission to have their picture(s) used for stock photos without compensation, they can sign a form that details the pictures being released. A separate photo release form is required for children and must be signed by the parent(s).
After the event pictures have been sorted and edited with any special effects, the photographer can create a PDF photo album to send to the client for approval or additional prints of pictures. By changing the security to allow only low-resolution printing, the photographer protects his or her revenue.
Photo of photographer in street via Flickr by Arenamontanus