The free tier accepts files up to 4 MB. If your ebook is larger — whether it's packed with images, has embedded fonts, or is a scanned PDF — you'll hit the limit. This guide explains why files get big, how to shrink or split them so they fit, and what Pro actually adds when you are converting in volume.
Why Your Ebook Is Over 4 MB
Most text-only ebooks are under 2 MB. Files over 4 MB usually have one of these characteristics:
Embedded images. Cookbooks, art books, and children's books pack many high-resolution images into the EPUB.
Scanned PDFs. A PDF created from a scanner is essentially a collection of images — easily 20–100 MB for a 300-page book.
Embedded fonts. Some publishers embed multiple font weights and styles, which adds size without adding content.
Lossless formats. CBZ and CBR (comic book archives) store full-page images, making them inherently large.
Free Method: Shrink Your EPUB Before Converting
An EPUB is just a ZIP archive with XML and images inside. You can reduce its size before uploading by stripping unnecessary elements.
Remove embedded fonts. If the ebook doesn't need custom fonts, extracting and re-converting to EPUB often strips them automatically.
Compress images. Open the EPUB as a ZIP file (rename .epub to .zip), find the images folder, and compress images with a tool like Squoosh or ImageOptim. Pack them back and rename to .epub.
Strip cover images. If you only need the text, remove the cover image from the archive before converting.
Reduce DPI in scanned PDFs. For PDFs, lowering the scan resolution from 300 DPI to 150 DPI can cut file size by 75% while keeping text readable.
Free Method: Split the Book Into Parts
If you can't shrink the file enough, splitting it across multiple conversions is often faster than waiting for Pro.
Split by chapters. Most reading apps can combine multiple EPUB files into a single library view. Upload each part separately, then merge the outputs in your reader.
Use Calibre to split. If you have Calibre installed, it can split a large book by chapter markers automatically. The free desktop tool handles files of any size.
Export as plain text. For AI prep (NotebookLM, ChatGPT), extracting just the text via EPUB → TXT gives you a small, manageable file with no images or formatting.
When to Upgrade to Pro
Pro does not raise the file size cap — 4 MB applies to every plan, Pro included. What Pro adds is batch conversion: several files in one upload instead of one at a time. Consider upgrading if:
You convert large files regularly. If compression and upload is a routine, batching it removes the repetitive part.
You need batch conversion. Pro allows multiple files in one upload, which matters when processing entire libraries.
You're a publisher or author. Managing several format outputs for one manuscript is faster with bulk uploads.
You convert the same book into several formats. Batch upload handles the repetition in one pass.
Quick Decision Checklist
- Under 4 MB? Upload directly. No action needed.
- 4–50 MB? Try compression (remove fonts, shrink images, lower PDF DPI).
- Over 50 MB? Split the file into parts — every plan caps one upload at 4 MB.
- Converting many files? Pro takes several in one upload rather than one at a time.
- Need plain text for AI? Convert to TXT instead — text-only files are almost always under 5 MB.
Key Takeaways
- 超过 4 MB 通常有四个原因。 内嵌图片、扫描 PDF（300 页轻松 20 到 100 MB）、多字重嵌入字体、以及 CBZ/CBR 这类无损全页图格式。
- EPUB 就是个 ZIP。 改扩展名为 .zip 打开，压缩 images 目录再改回来，体积立刻下来。
- 扫描 PDF 降 DPI 是最划算的一招。 从 300 DPI 降到 150 DPI，体积减 75% 而文字依然可读。
- 4 MB 上限对所有套餐都一样，Pro 也不提高。 Pro 增加的是批量上传，不是单文件额度。
- 超 50 MB 就分册。 按章节切开分别上传，多数阅读应用可以把多个文件合并成一个书库视图。
- 纯文本几乎都不到 5 MB。 喂给 NotebookLM、ChatGPT 这类工具时，转 TXT 比转 EPUB 省事得多。