Compress books for Kindle

Document Too Large
Send-To-Kindle does not support documents over 50MB.

I use Kindle app to read books on my iPhone. When I was trying to send a large .mobi file to Kindle, the above error occurs. The mobi file was downloaded to the stock Files app on iPhone. Then I tried another big PDF file which is more than 50MB and saved in my iCloud account, still no luck. It seems we can’t transfer or import documents that exceed 50M to Kindle app on iPhone. In this case, you can use a third-party iPhone manager program which can help you copy books to Kindle Documents folder directly. Check out this guide to transfer books from computer to Kindle app on iPhone iPad. Another workaround is to reduce the file size of the books. In this article, we will show you different ways to compress large books so you can import them to Kindle app on iPhone iPad.

Compress Mobi to ePub, AZW, PDF

Ebooks in .mobi format are usually bigger than those in .epub, .pdf or azw formats. One of the reasons is that mobi files contain multiple versions of the ebook for all the different kinds of kindles. When reading mobi books using Kindle app on iPhone, we can convert mobi books to ePub, AZW or PDF to reduce down the file size.

Calibre is a great book converter and editor for both Windows and Mac users. We can use Calibre to convert the .mobi to .epub formats for iPhone iPad. Best of all, it is totally free to download and use.

Convert .mobi to .epub book using calibre on Mac

Compress images in books

High quality images is one of the main reasons why your books are so big. If you can reduce the size of images in an epub, azw or mobi file, the file size of the whole book will be reduced down largely.

Calibre provides options to compress images in epub and azw books. Add the books to the editor, click Edit Books from the top menu bar to open the book in a new window. Choose Tools from the top menu, select Compress images losslessly from the drop down menu.

calibre tools - compress images losslessly

Lossless vs lossy compression

You can compress the images in the books lossly, reducing the file size of the book, without affecting image quality. Typically image size is reduced by 5% to 15%.

You can also click to enable the option ‘Enable lossy compress of JPEG images ‘, then set a value of the compression quality. This can largely reducing down file sizes of the images and books.

Compress PDF books

In some related articles, we discussed several ways to compress PDF files on computer. So if you are trying to send large PDF to Kindle but failed due to the large file size limit, you can simple compress them first. See this guide to reduce PDF file size using Preview on Mac, and this tutorial to compress PDF files on Windows PC and Mac.

Transfer books to Kindle or iPhone iPad

Once you are done with the book compression using Calibre, and the book size is smaller than 50MB, you can make use of Content Server, a built-in feature with Calibre, to send books from Calibre to iPhone iPad wirelessly. Then copy the books to Kindle app, the stock Books app or other ebooks readers you have on your iPhone or iPad.