Apps similar to Panels - Comic Reader include SumatraPDF, which is free and open source. Other options are Okular, KOReader, PDF-XChange Editor and Koodo Reader. PDF Readers like Panels - Comic Reader are a crowded field, with more than 100 alternatives for Windows, Android, Linux, iPhone and iPad.
Uviewer is a modern WinUI 3-based integrated viewer for Windows, specialized for reading Manga and Light Novels with support for images, archives, and Aozora Bunko.
South Korea
Xpdf is a free PDF viewer and toolkit, including a text extractor, image converter, HTML converter, and more. Most of the tools are available as open source.
United States
DjVuLibre is an open source (GPL'ed) implementation of DjVu, including viewers, browser plugins, decoders, simple encoders, and utilities.
ACBF Viewer is a viewer application capable of reading ACBF, CBZ/CBR and ACV comic book file formats. It is written in Python programming language. Currently there are versions for Linux, Windows and Android (ACBF Viewer for Android). It is distributed under GNU GPL version 3.
Cloud-based PDF viewer and annotation tool for your browser. Perfect for Chromebook 1:1 schools. View, export, print and share your PDF files straight from your browser. Kami works on all browsers and works best with Google Chrome.
Comic Seer is a freeware sequential image viewer desktop application for Microsoft Windows and Linux, specifically used for viewing Comic Book Archive files containing image formats, such as JPEG, PNG, TIFF, and others.
DinoReader is built on the FBReader reading engine, but includes features not currently available in the mainstream app:
United Kingdom
Shelf is a convergent document viewer that supports PDF and EPUB files. With Shelf, you can organize your document collection by creating tags, like other Maui applications.
Mexico
New Zealand
