Sideloading & Formats

Every e-reader has a way to load books that didn't come from its store. The 2026 picture is healthier than it's ever been: Kindle now accepts EPUB, Kobo has always taken EPUB, and the Android e-readers (Boox / Bigme / Daylight) take everything. Pair with Calibre & Format Conversion, KOReader, and Self-Host Libraries.

Format primer

• ★ EPUB — open standard; the default; everything reads it except old Kindles.
• ★ KEPUB — Kobo's enhanced EPUB; gives Kobo proper paging + stats; convert with kepubify.
• AZW3 / KFX — Amazon's; what your Kindle stores; you don't author these — Calibre / Send to Kindle convert to them.
• MOBI — old Kindle format; deprecated by Amazon since 2022; convert to EPUB or KFX.
• PDF — fixed-layout; fine on big screens; reflow with KOReader on small ones.
• CBZ / CBR / CB7 — comics / manga archives.
• DJVU — older scanned-book format; KOReader supports.
• M4B — audiobook; chapters + cover. See Audiobook Self-Host.

Sending to Kindle

• ★ Send to Kindle (web + email) — free; accepts EPUB / DOCX / PDF / TXT / HTML up to ~200 MB since 2024; the official sideload channel.
• Send to Kindle (Mac / Win desktop) — free; drag-drop.
• Calibre + Send to Kindle — free; Calibre converts + emails for you.
• Push to Kindle (Five Filters) — free; for web articles.
• Email-to-Kindle — free; every Amazon account has a @kindle.com address; whitelist senders.

Sending to Kobo

• ★ USB drop-in — free; just copy EPUBs to the Kobo's Books/ folder.
• Kobo Sync via Calibre-Web — free; pretend to be kobo.com; serves your library.
• Pocket integration — built-in; saves articles to your Kobo.
• Dropbox / Google Drive integration — paid Kobo Plus or app-side.
• kepubify — free; convert EPUB → KEPUB for full Kobo behaviour.

Sending to Boox / Android e-readers

• ★ Google Play / F-Droid + Kindle / Kobo / Libby / KOReader apps — free; full Android.
• BooxDrop / OneDrive / Google Drive — free; built into Boox firmware.
• Syncthing — free OSS continuous sync; the power-user pattern.
• OPDS via KOReader — free; subscribe to your Calibre-Web / Komga server.

Sending to reMarkable

• Drag-drop in the desktop app — free; PDF / EPUB.
• Read on reMarkable (Chrome extension) — free; web → reMarkable.
• Calibre rmcloud plugin — free; community.
• rmapi / rmfakecloud — free OSS; CLI / self-hosted reMarkable backend.

OPDS (the FOSS sideload protocol)

• ★ OPDS is the "RSS for ebooks" — every FOSS server (Calibre-Web, Komga, Kavita, COPS) exposes an OPDS feed; every FOSS reader (KOReader, Aldiko, FBReader, Moon+, Panels, Chunky) consumes one. Sign in once, browse your whole library, download direct.

Wireless device protocols

• Calibre Wireless Device API — free; KOReader / Calibre Companion talk to your library over Wi-Fi.
• Calibre Companion (Android) — paid; pulls from Calibre on your desktop.
• OpenDS / OPDS catalogues — free; cross-platform.

Patterns

• ★ EPUB everywhere — keep masters; convert on send.
• One library, many devices — Calibre / Calibre-Web central; Kindle / Kobo / Boox / reMarkable / phone all read from it via OPDS or send-to-device.
• Don't fight the format — let Calibre convert; don't hand-edit AZW3.
• Backup My Clippings.txt — Kindle stores all your highlights here; one USB cable + one copy = lifetime archive.

Pick this if…

• Sideload to Kindle: Send to Kindle (email or web).
• Sideload to Kobo: USB drag-drop or Calibre + kepubify.
• Sideload to Boox / Android e-reader: install KOReader / Kindle app from Play.
• Sideload to reMarkable: desktop app drag-drop or Read on reMarkable extension.
• Library protocol that everything speaks: OPDS via Calibre-Web / Komga / Kavita.