Every document created in InstaPDF has a unique public document ID associated to it. If you choose to share a document link with a third party, the associated document's PDF can be downloaded - depending on the privacy setting of the document.
This means even when you share a link, documents can be made inaccessible when changing the document to "Private".
By default, all documents are set to have a publicly accessible link. However, this setting can be changed in "Accounts -> Security & Privacy" in the iOS app. We plan to change this setting to apply to other clients, i.e. Mac, in the future. Please note: the unique document ID is truly unique and incredibly difficult to "guess", therefore, unless you share a link, documents will stay private.