I feel like the story itself has a different feel depending on if it's read with illustrations or without. The PDF was a good opportunity to give folks another choice in the way they first want to read the story and create their own impressions of characters before they actually get to see them in the book (or keep/maintain their own ideas of what the characters/events that take place in the story feel and look like to them)!
All PDFs don't include illustrations! It gives readers a chance to decide if they want the experience without my input (how I imagine the characters/setting/moments to look). OR even compare the two by reading the PDF with no illustrations first and then reading the books with illustrations.
Don't see the answer to your question? Ask the project creator directly.