The only way I know to get expandable forms (as in a PDF document that will reflow/expand as the content inside of it grows) is with Adobe's LiveCycle Designer product. This is a Windows-only application that provides an XML-based PDF format which allows for content reflow and expandable forms. You can use LiveCycle Designer without needing or using the rest of the LiveCycle Enterprise products.
To create a re-flowable form, make sure your document is saved as a Dynamic XML PDF format and then organize your form into sections of subforms (see Adobe's help page). This helps position elements in relationship with each other. Configure subforms that you want to "expand/reflow" by going to the Object Palette for the selected subform and making sure it's set to "Flowed". Again see Adobe's help manual for LiveCycle Designer. Also make sure your form elements (i.e. your text input areas) inside these subforms are configured to expand with the content. Then save and test to ensure it works.