PDF compressed when sent with Calibre Wireless Connection

Hi,
I'm trying to send PDF's (that are made of images only) to my Kobo device with Calibre Wireless connection. I drag the PDF into Calibre and then send to device in Koreader latest version. In my Kobo Aura 2nd Edition the file size has changed. Also sometimes I get blank pages later in the PDF.

If I copy the PDF directly using the USB wire it works fine. How do I turn off the compression when sending with Calibre?

This is how I do it right now:
1. Drop PDF (made of images Jpeg) from computer into Calibre Latest Linux Mint 18.
2. Turn on Calibre Wireless Connection
3. In Latest Koreader for Kobo Aura I press connect in Calibre Wireless Connection.
4. In Calibre, click send to device then send to main memory.
5. PDF is now in folder on Koreader, but the size is now smaller.

Why does the PDF change size?

(Update:)
Seems to be fixed for me:
1. Turn on Calibre Wireless Connection
2. Connect Calibre Wireless connection in Koreader
3. When Koreader recognized in Calibre, go to Device then configure this device
4. Cover Thumbnail compression quality set to 99 stops my PDF errors.