Neat. Another use case for which you might want to think about a sample is extracting data from filled PDF forms. (That use case is why I once had to write a PDF parser.)

Since you read&write, maybe also a use case of programmatically filling some form fields in an editable PDF form. Such pre-filling some of the fields for a particular Web site user in a dynamically-modified PDF form they download. But the source PDF form can be hand-crafted and maintained separately, like people often want to do, not generated from scratch by your code.