No problem! Well, maybe… Depends on what you consider a problem to be…
Anyways, here's a workflow that reverses the order of pages in a PDF. One caveat is this converts the pages in a PDF to images. However, since the PDF you have is a document of scanned pages, it doesn't matter because they are images anyways. Well, maybe, some software offers other options, but you'd likely know what you are doing. 😉
Anyways, it's a simple four-action workflow. I know, looks complicated, but that's just because I'm explaining it way too much. :-p
*1. "Get Specified Finder Items"* - Feel free to use "Get Selected Finder Items" if you want, just get the PDF into the workflow somehow. You can drag and drop the file into the workflow and this action will be added for you.
*2. "Render PDF Pages as Images"* - For full quality, choose a lossless image format such as TIFF. The compression option doesn't matter for the TIFF option. If you use JPEG or JPEG 2000, the compression should matter, but I didn't notice any difference between least and best.
For the resolution DPI, choose whatever DPI at which you scanned the pages. This is likely 300 DPI for text, or 400 DPI if you're concerned about scanning really small fonts and plan on doing OCR sometime later.
The color model should be the same as your scanning settings. Likely, this will be Gray if you didn't need the color because it'll be a much smaller file size for the scanned images, and therefore the PDF.
(By the way, the images will located in some temporary place so you don't have worry about where they are. Normally, this action would be followed by a "Move Finder Items" action because of that, but we don't need them after the new PDF has been created from them. Also, a "Rename Finder Items" action tends to follow it because of the random names given to the files, but again there is no need since we don't need them anymore.)
*3. "Run AppleScript"* - This is the magical part. When you add this action you will see some text already in the action. There is one line in the middle that looks like this:
<pre> (* Your script goes here *)</pre>
Simply replace that line with this nifty-keano line:
<pre> set input to reverse of input</pre>
In a nutshell, it simply takes the list of images files and reverses their order. Boom!
*4. "New PDF from Images"* - Change the location if you want, and set the name if you like. No need to change the setting managing the size of each page since they are very likely all the same size. Play with that setting some other day when you got different sized images.
And that's it. If you want to do everything in one workflow (and you've already tested this part to make sure you get the desired results), you can add the PDF of odd pages at this point. Just drag and drop it into the workflow here. Though, this will make the odd paged PDF second, so you'll want to have yet another "Run AppleScript" action just like the one you've already got so as to reverse the order of these two files and put the odd paged PDF first in the listing. Simple select the "Run AppleScript" action in your workflow and copy and paste it. When you paste it, it'll automatically be appended at the end of the workflow. Convenient, eh?
At this point you'd have that other action you're using, the one that shuffles the PDFs together. However, unless you're trying to make a workflow you intend to use often, I wouldn't worry about trying to make it all happen at once. Two separate workflows are just fine for a one time use.
