If we like to swap page from the same PDF file we could do something like this.
If you run the script twice it will return to original order.

pageAtIndex{1,2} first page in PDF is 1

Applescript:

use AppleScript version "2.4" -- Yosemite (10.10) or later
use framework "Foundation"
use framework "Quartz"
use scripting additions

set thePath to POSIX path of (choose file)
set theURL to current application's |NSURL|'s fileURLWithPath:thePath

set savePath to POSIX path of (path to desktop) & "newSavedFile.pdf"
set saveFile to current application's |NSURL|'s fileURLWithPath:savePath

my swapPageWith:theURL pageAtIndex:{1, 2} savePath:saveFile

on swapPageWith:theURL pageAtIndex:{a, b} savePath:saveFile
   set pdfDoc to current application's PDFDocument's alloc()'s initWithURL:theURL
   
   set pageCount to pdfDoc's pageCount()
   if (a is greater than pageCount) or (b is greater than pageCount) then return
   try
       if (pdfDoc is not missing value) then
           (pdfDoc's exchangePageAtIndex:(a - 1) withPageAtIndex:(b - 1))
           (pdfDoc's writeToURL:saveFile)
       end if
   end try
end swapPageWith:pageAtIndex:savePath: