Content cannot center in PDF converted by Calibre

Hi all,

I tried to convert my books from EPUB to PDF. The margin on the right is much bigger than the one on the left, despite I have inputted identical value as the following:

https://www.howtostudycantonese.com/...20;3.42.15.png

See this image as the margin is not balance:

https://www.howtostudycantonese.com/...20;3.46.45.png

How can I fix this issue? Attached the Ebook file. Thank you so much for your help.