@Suzanne93
You can upload images and documents from your app, but not on your computer. However, these attachments sent from the app will show on the booking message board when viewed on the computer.
For me it's worked best to integrate my info into images, sometimes by making selective screen shots of documents, sometimes just images to explain instructions. I send my arrival instructions for ex. by email, but also through the app, so that guests have everything accessible on their phone.
Hope this answers your question.
Not sure if that answers your question re. the entry authorization form...