A few months ago, Firefox for Android and iOS was added to the list of apps that can open PDF. Although there are thousands of PDF viewers in the market, I found Firefox very convenient for local document management on Android due to its support for shortcuts and collections. The URIs pointing to the emulated storage are persistent. So when the URIs of PDF files are sent by any file manager to Firefox, they can be added to collections, shortcuts, and bookmarks. Personally, I found collections more useful for local document management. However, Firefox for Android only opens PDF. Although other MIMO types such as images, videos, audio, and text/html can be easily opened in Firefox, and these files can then be added to collections, shortcuts, and bookmarks.
Without even checking the code, I'm pretty sure that no development is required to get Firefox to open local images, video, audio, and text/html, just some simple extra coding in the manifest and, if I'm not mistaken, in the onCreate method.