Find out resolution of screen of your device. Than...
In command line menu (terminal window) add these numbers.
-w (your device width)
-h (your device height)
-dpi 150 (for example)
Example:
k2pdfopt -w 600 -h 800 -dpi 150 name_of_your_file.pdf
If you are using GUI version of app, than make sure that this
crucial data is entered into gui of application.
I think that gui version allready has your device in settings.
I am using terminal version (on Debian Linux x64).
You can find many information on all available commands in web link
https://www.willus.com/k2pdfopt/help/options.shtml
If you need more data, you can sand me private message. Greetings!