When I use the Crop Pages feature under Organize tab in Foxit PhantomPDF to bring up the Set Page Dimensions window, I find that some page sizes listed in Fixed Sizes drop-down box are not selectable as shown in the screenshot below.
I received the error message as shown in the screenshot below when I chose to create custom page Width and Height under Change Page Size part in Set Page Dimensions window.
The purpose and usage of Change Page Size part in Set Page Dimensions window is only for increasing page size. It is used to enlarge the white margins of PDF page. You cannot use the Change Page Size entry to reduce the page size. So, the page sizes listed in the Fixed Sized drop-down options must be larger than the original PDF page size. For example, the current page size of the PDF file you are opening is A0, you could find that all pages sized which are smaller than A0 (such as A0,A1,A2,A3,A4, etc.) will be grayed out.
When you want to manually input custom page Width and Height value, you also need to input them to be larger than the current page width and height. Otherwise you will receive the following error message:
Page size may not be reduced.
To reduce page sizes, you have to use the margin controls entry in Set Page Dimensions window.