I think you only have a few options here. Two of which would be:
- Change the layout in InDesign to be facing pages - 1 page for 1 page - then export PDF again
- Open the existing PDF in Acrobat and use the Crop pages tool. Before cropping though you will need to copy the pages. Eg. if 1 PDF page has 3 brochure pages, you would make 2 copies and crop each of the 3 pages to size. Including guides in the original PDF would help here.
As I read this, there are two pages that are essentially a landscape spread on one page, with two columns, one for the left and one for the right page content on each side of a single sheet.
If that's the case, I think the easiest thing to do is create a new facing pages file in ID that is 4 pages, each half the width of the current page, then copy/paste in place the content from the original to the new file.
One of your current pages is the front and back of the folded brochure. You'll need to copy the halves of that separately, but the other page is the inside spread and you can do it all at once.
You are correct about the landscape spread & folded brochure, and I thought about copying & pasting into a new page but because we have 5 different versions (English, French, Chinese, German, Japanese) and we frequently update these brochures, I would have to repeat the steps each time and would probably end up with ther same amount of work as cropping the PDF.
You didn't say what version of InDesign you're using. If you're using InDesign CS6 or CC, you could use the Content Collector and Content Placer tools and create links between items on each page. That makes it easy to handle updates.
Frankly, I'd recreate it anyway and abandon the other version. You can even set up two two-page spreads if you turn off page shuffling so it looks the same in ID.
CS5.5, but I will check this out in the demo version of CS6.
I will discuss this further with the graphic artist.
Thanks everyone for your comments and suggestions, you have been very helpful.