Free tools · Ebooks and book files
Book file optimiser
A 90 MB EPUB is usually a few pictures far bigger than any screen shows, or a file saved with nothing compressed. This makes it smaller without changing what readers see, then runs the EPUB checker on the result. It also makes the photos in a print PDF smaller (over-sharp ones to 300 ppi, lossless ones as high-quality JPEG), and brings a cover picture under a size limit, keeping the same pixel size.
On KDP, a 70% royalty ebook pays a delivery cost for every megabyte (the KDP royalty calculator shows it), and smaller files download faster everywhere. Done in your browser; nothing is uploaded. Free for members.
No file to hand? (an EPUB of three chapters from The Signal by R. S. Breed).
What it changes, and what it doesn't
Pictures bigger than the size you choose, or over 4 million pixels (the most Apple's "Prepare your book" page allows; the cover is left at its size), are scaled down, keeping their shape and their format (a transparent PNG stays a transparent PNG), and a picture is only swapped if the new one is clearly smaller. Text files are compressed. Leftover computer files (.DS_Store, Thumbs.db) are left out. Your words, fonts, links, file names and the order of the book aren't touched.
PDFs
In a print PDF, only colour photos change, in two ways. Photos printed sharper than you choose (450 or 350 ppi) everywhere they appear are brought down to 300 ppi, which is what KDP asks for. Photos stored losslessly, which some programs do and which can make each one several times the size, are saved as high-quality JPEG; pictures with few colours (charts, drawings, flat colour) stay lossless. CMYK and greyscale pictures, masks, fonts, text and everything else stay exactly as they are. The new file is read again and compared with yours page by page, and it's only offered if nothing else changed. If a PDF still needs to be smaller, export it again from the program that made it, with pictures at 300 ppi, and check it with the print file checker.
Written a book? List it free.
Your cover, blurb and store links, a first chapter readers can try, and numbers on where they stop. Host your advance copies free too.