Extract versus delete
If you need fewer than half the pages, extracting is easier; if you need most of them, delete the extras. Both produce a new file and leave the original untouched.
Extracting with the split tool
In Split PDF choose Extract page range and type pages with commas, such as 1,3-5 to get pages 1, 3, 4 and 5. Or choose Split every page to get one file per page inside a ZIP.
Deleting pages
In Delete PDF Pages type the pages to remove such as 2,5-7 and download the result. You cannot delete every page.
Reordering
After extracting you may want a different order; use Organize PDF Pages and type the new order, and repeat a page number to duplicate that page.