Depending on whether you need a lightweight viewer or a full-featured library manager, these are the top choices: 1. Sumatra PDF (The Lightweight Choice)

Supports EPUB, PDF, MOBI, CBZ, and CBR formats.

Unlike standard desktop apps, portable readers are "self-contained".

Sumatra PDF is widely considered the fastest portable reader available.

Portable EPUB readers allow you to carry your entire digital library and reading software on a single USB drive or cloud folder, ensuring you can read on any computer without needing administrative rights for installation. Why Download a Portable EPUB Reader?

The .exe file is roughly 7 MB, making it launch almost instantly.

Advanced users can tweak the EbookUI in settings to change font sizes and background colors. 2. Calibre Portable (The Powerhouse)

They leave no traces (like registry entries or temporary folders) on the host computer.