OpenDocument Reader is an app for Android and iOS for opening and lightly editing office documents. It is built around the OpenDocument format (ODT, ODS, ODP) used by LibreOffice and OpenOffice, and also opens DOCX, XLSX, PPTX and PDF, among others. Everything works offline, without an account and without uploading documents: files are processed on the device itself.
The app is maintained by Stefl und Taschauer OG, an Austrian company; its website describes the project as small and independent. The rendering engine, OpenDocument.core, is a C++ library under the MPL-2.0 licence, and the source code of the apps is public as well (MPL-2.0). There are three editions: a free edition with advertising, a paid edition without ads, and a Foss edition distributed through F-Droid and GitHub. According to the privacy policy, the ad-supported edition uses Google AdMob. Because there is no server behind the app, your documents stay on your own device.