I'm not sure why you would want to do that, but I imagine there are reasons. A multiple Selection of PDF files can be copied to the Clipboard and Pasted into Pages in a batch way. What apparently isn't supported is your desired importing the multipage PDF, and I can't tell you why.
Perhaps someone else knows of a procedure to get around this problem.
Could you consider assembling your document in Preview? If so, Export your Pages document to PDF and merge it with your multipage PDF document in Preview.
I will try to be more precise.
I have a multiple page document that I scanned into a pdf format.
I want to be able to view - not edit - it on my iPhone via iCloud.
I do not know of any way to get a pdf up to my iCloud so I thought that if I could convert it to a Pages document that I could then upload it (the Pages document) to my iCloud and thus be able to view it on my iPhone.
So what I really want to do is be able to view a pdf document that is on my iCloud on my iPhone 5.
Perhaps my question should have been - How do you get a pdf document up to iCloud?
I agree dropbox is a good way to go. If you are looking for just a quick and dirty way to view the PDF whenever you want on your iPhone, you could just email it to yourself. Once you have the email open the attachment and then tap open in and select iBooks. Now the PDF will always be on your phone.