The way files are passed is the same way the files are merged…you can reorder before passing the file list to order them as you need
as far as bookmarks is considered what is that you are looking at? ideally this might not create any bookmarks and there is no straight forward activity that might help with bookmarks
Try the below way. This will take the PDFs present in the folder in an ordered way.
pdfFiles = Directory.GetFiles("YourPdfDirectory").OrderBy(Function(pdf) pdf).ToArray()
Join PDF files acitivity
Note: pdfles is of DataType Array(System.String)
As far as creating bookmarks there isn’t a direct activity that might help to create bookmarks. The above query will help you join the PDFs in an ordered way. Replace YourPdfDirectory with your folder path which contains the pdf files.