Best, duplex enabled way to merge PDFs

Question

I'm merging multiple multi-page source PDFs into one new result PDF for customers to print.

Now some source PDFs contain an even number of pages, some contain an uneven number (unpredictable).

Some customers print simplex, some print duplex. This is difficult because the simplex customers don't want to have empty pages between the documents and the duplex customers don't want to have and end-page and a start-page on the same sheet.

What's the best way to offer a good experience for both types of customers?

Is there a PDF feature for marking document borders? I couldn't find anything...

[Edit]

To further clarify my problem: People upload pdf documents to my tool, merge them into one and download them again. From a software point of view i am completely unaware of their printing configuration/habits/setup/devices. Thus i seem to need a PDF feature for storing the "document borders" or "printing instructions" (document 1 goes from page 1-3, document 2 goes from 4-11, ...) - but this feature does not seem to exist - or something else that has the same effect and can be stored in the file because that file is all the software produces.

[Edit 2]

An obvious solution to this problem would be asking the user if we wants to have blank pages inserted after every single merged document with an uneven page number (except the last one), but this would ruin the digital reading experience of the PDF document.

Answer 1

There is no feature in the PDF specification for "sub-documents". A PDF document is an array of pages. If you are joining them together, then you are making one document of all the pages from the source documents.

It might be possible to use Adobe's Job Definition File format (JDF) to contain data describing the sub-document boundaries (as it's extensible XML). A JDF file can be stored within a PDF. However, your users would need software at their end that can parse the JDF file and act accordingly.

Alternatively, you could create two separate tools: one that adds blank pages to each source document with an odd number of pages, and one that doesn't. However, this would rely on your users exercising their judgment to select the correct one.

Another course of action might be to advise those users with duplex printers that there's little merit in combining the PDFs, and that they should just send multiple PDF documents to their printer.