What's the meaning of "first-page-first order"?
I am pretty sure this is just a ridiculously dated instruction. On some older printers, the pages were ejected print side up and new pages were added to the top. This means that the first page printed would end up being the bottom page of the document when you picked it up. If you then wanted to staple the document in the correct order, you needed to reorder everything. With these printers it was much more desirable to print "back-to-front" such that when you pick up the document from the printer everything is in the "correct" order. Some computers provided (and maybe some still do) the option to print "back-to-front" to avoid this. Some people conceivably, though I have never seen it, produced documents in a first-page last order to help with the printing. I think this instruction just means that the pages should go in the order you expect them in: the first page of your pdf should be page 1 and the last page N. In a first-page-last order the first page would be N and the last page 1, which would be really awful to read on screen.