File Types, Sizes & Formats

Everything you need to keep your X3 or X4 library fast, clean, and properly rendered — for both devices.

2026 Edition · Applies to X3 & X4

On this page

TL;DR

Best book format — clean EPUB in UTF-8
OK book format — TXT in UTF-8 (simple files, no huge mega-files)
Images / covers — JPG or BMP, ideally around 480×800, grayscale-friendly
microSD — exFAT or FAT32, simple folders, no crazy nesting
Conversion tool — Calibre for cleanup before sending to your X3/X4

Supported formats

Both the X4 and the X3 are simple e-readers: built to display text and basic images, not to render complex HTML engines.

Recommended core formats

EPUB

Best choice for most books. Supports chapters, table of contents, and basic styling. Plays nicely with Calibre for pre-processing.

TXT (UTF-8)

Great for lightweight notes, articles, and simple books. No layout, no styling — just pure text. Encoding matters: always use UTF-8.

Image formats

JPG

Good for covers and wallpapers. Use grayscale-friendly images; avoid ultra-compressed artifacts.

BMP

Slightly heavier, but fine for static images, logos, or simple graphics.

The devices are not meant for heavy image books or comics. Occasional images are fine; pure text is where they shine.

File sizes and performance

The X3 and X4 can technically open very large EPUBs, but performance drops as file size and complexity grow.

For EPUBs

Try to keep EPUBs under 20–30 MB when possible.
Avoid huge "everything I've ever written" collections in a single EPUB.
Split gigantic series into several volumes (Book 1–3, Book 4–6, etc.).
Prefer a handful of well-prepared books over hundreds of messy ones.

For TXT

Short, focused TXT files are better than one giant file with 10+ novels.
Split by book or topic so navigation stays sane.

Folder structure on the microSD card

A little structure makes a big difference. The filesystem doesn't need anything fancy.

/Books
  /Fiction
  /Non-Fiction
  /Essays
  /Short-Form
/Reference
  /Language-Learning
  /Technical

Avoid very deep nesting (more than 2–3 levels). Keep folder names short and clear.

If you use both X3 and X4 with the same card, consider splitting:

/X3-Short-Reads — essays, language learning, articles
/X4-Long-Reads — novels, non-fiction, long-form

Image and wallpaper sizing

The screens are small and e-ink has limits.

Target resolution

480×800

Both X3 and X4 work well with images sized around 480×800. The X3 has a slightly smaller physical screen but the same logical reference resolution.

Best practices:

Use grayscale-friendly images with strong contrast and no tiny details.
Avoid very dark images with subtle gradients — they don't translate well on e-ink.
If an image looks noisy or muddy, reduce detail and simplify shapes before exporting.

Text encoding and languages

If you see weird squares, question marks, or broken characters, it's almost always an encoding or font issue.

Make sure TXT files are saved as UTF-8.
When converting with Calibre, set input and output encodings to UTF-8 where possible.
Avoid double-conversions (e.g. DOCX → PDF → EPUB → TXT).

For non-Latin scripts or heavily accented content: use fonts that explicitly support your language, and test one sample book before batch-converting your entire library.

Calibre workflow: cleaning your EPUBs

Most ugly layouts and spacing bugs come from messy EPUBs, not from the reader. A simple pre-flight routine in Calibre:

Import the book into Calibre.

Right-click → Edit book.

Run: Check book / Repair book to fix obvious errors.

Run: Beautify all files to clean up the HTML.

Run: Remove unused CSS to slim down themes.

Optionally: remove custom fonts that don't add real value; standardise margins if extreme.

Save and export the cleaned EPUB.

This takes a few extra minutes but prevents many layout issues, especially with older or heavily converted ebooks.

FAQs

Can I use PDF?

The devices are not optimised for complex PDF rendering. Simple text-only PDFs may work, but for anything serious, convert to EPUB first using Calibre.

Do comics or manga work well?

Only with compromises. The screens are small, there's no pinch-to-zoom, and e-ink refresh is slow. These readers are designed for text first.

What microSD card size should I use?

Moderate sizes (32–128 GB) are plenty for ebooks and keep scanning times reasonable. Extremely large cards packed with thousands of files can slow things down.

Can I safely delete files directly from the card?

Yes, as long as you safely eject the card from your computer afterwards. If the reader behaves strangely, rebuild the index or restart the device.