You can use this app, without any data about your person being stored or processed online. The processed data of the document is only locally and offline persisted on your device and is deleted by removing the document from the app or by uninstalling the app. Better safe than sorry! That's why the Source code of the app is fully Open Source and therefore publicly available.
The chosen PDF document is copied into the app's storage space and is solely read out for display. The app does not sensually grasp the document's content. I.e. for the app it makes no difference if you e.g. import a health related certificate or a bus ticket. To enable the fullscreen display of the QR code, the document's 1. page is searched visually (not textual), if there is any QR code inside. The text content of the QR code is read out by the app at runtime and soleley used for creation of the fullscreen QR code. All data is saved locally and offline on your app on your respective device. The document's data does at no point leave the app and can be removed from the app storage at any time, by deleting the document with the delete functionality of the app or by uninstalling the app. Other apps have no access to the document in the app's space. No ads, no tracking.
No, your data will not be passed on to third parties.
After inserting the password for decryption, the document stays permanently (until deletion by the user) decrypted in the app's storage. Your password is solely used for decryption and is not saved anywhere!
This app requires no permissions at all. Therefore it has also no access to things like internet, camera, or other files on your device, etc. The app can solely access the currently imported documents.