You can upload, fill out, sign, and send a document using the HelloSign mobile app. Please keep in mind that the current HelloSign mobile app is separate from the HelloSign web browser app, so the two different apps will not sync with one another. For example, a document you send using the mobile app will not appear in your HelloSign web account if you have one, and vice versa.
Both of our HelloSign mobile apps require documents to be in PDF format in order to upload, fill out, and sign. So first, make sure the document you are trying to retrieve is in PDF format. If it isn't, that's why you aren't able to select it.