Documents allows you to work with a PDF file while viewing two pages at a time, which makes your workflow smoother. To find the option in Documents on your iPad, simply open the needed file > tap the 'aA' icon in the upper right corner > select the 'Two Pages' option. As soon as you tap the option, you will get an additional feature - the ability to show the first page alone:
To use the same mode in Documents on your iPhone, please open a file > tap the 'aA' icon in the lower right corner > tap View Settings (aA) > select the 'Two Pages' option.
Here you can also toggle on the 'First page Alone' option.