I believe every download must be a separate file. So you need a separate .pdf for the parts together, then individual pdfs for each part.
CompleteParts.pdf
Fl_I_II.pdf
....
Cb.pdf
Then upload
everything together. On the AddFile/Submission page, don't merge them: use "Drag or Click to upload as separate files".
For reference, I used the Pictures At an Exhibition Page:
https://imslp.org/wiki/Pictures_at_an_E ... 2C_Modest). Go to Arrangements and Transcriptions. The second Ravel orchestration (#78809) is in the format you want. To see how it's done
1. Edit the page (Top menu> Page> Edit). This is the complete work page, so
2. Scroll down to ===Arrangements and Transcriptions===, and then the Ravel orchestration:
=====For Orchestra (Ravel)=====
{{ [Information about score #741261] }}
{{ [Information about score #78809] }}
Every choice in the download list is a separate file. The first file shows up in blue next to the download arrow, and the remaining items are listed below it "→29 more". In this example, the first file, "Complete Score" is the Conductor's score, but in yours it will be all the parts bundled together in a single file, and called "Complete Parts"
4. Keep the Ravel page (or the reference page of your choice) open in Edit mode. Once you've uploaded everything together, Edit the work page, and fix any mistakes by copying the style from the reference page.
I find it easier to keep two copies of the reference page open -- one in normal mode, the second in Edit-- so I can see what the Markup looks like on the final page. Hopefully someone more knowledgeable can point out any mistakes I've made here.