JellyBook is a book and comic reading app for Jellyfin. Its a app thats meant to allow you to read your files from Jellyfin on mobile. Supported file formats include:

.cbr & .rar: cbr (Comic Book aRchive) is a proprietary file format so is not necessarily encouraged.
.cbz & .zip: another Comic book format
.pdf: the Portable Document Format as you know it
.epub: Electronic PUBlication is a wide-spread eBook format
More formats will be added over time.