I had this same request from users a few times a year and finally figured out a couple solutions suitable for our needs.
The simplest method is to use the Combine Files function in Acrobat Pro. Add the portfolio to the list, Acrobat will prompt with a warning that "The files within the PDF Portfolio have been added to the file list" and the individual files can be combined as normal.
Upside: This is a faster and easier method. Skips needing to export first (and deal with duplicate filename collisions).
Downside: The order of the individual document/emails will be alpha sorted by the filename/subject, unless manual reordered in the Combine Files list. Attachments are also lost.
If date order and attachments are needed, which is what my requirements called for...
The only method I have found so far for preserving both the order and attachments is to manually print each individual email, with the print attachments options enabled, to the Acrobat PDF virtual printer, then combine.
Configure the virtual printer to auto save all jobs to a folder without prompting. This will auto rename to unique (but meaningless "Memo Style (#).pdf") filenames. Set this virtual printer as the default printer.
Outlook will prompt for most attachment types before printing. Outlook has to be run as admin to be able to have the option to suppress the "are you sure you want to open this file type" popups for each attachment type.
The first gotcha for print from Outlook is that the current version at time of writing is inconsistent with keyboard focus and ctrl+p then enter will not just work 100% of the time. Each time the print screen is opened, there is a chance the keyboard input will not be on the print button but focused on the "settings" control. So pressing enter after ctrl+p will not print but instead switch from "memo style" to "table style", which will then be the default till switched back.
The second gotcha is that if multiple emails are selected at once and printed, all of those emails will be printed as a single PDF and the attachments will be printed afterwards individually, separating them from the email they came from. If you want each email to be followed by its attachments, the each email with attachments will need to be printed individually.
Due to the to the inconsistent keyboard input behaviors of Outlook, automating with something like Autohotkey will be tricky. It may be possible to automate this via VBS scripting, but I'm not sure how much of that functionality remains in the current version and is unlikely to be supported in "New Outlook".
Once all emails and their attachments have been process through the virtual printer, sort the output folder by date, then drop all the PDFs in to the Combine Files workspace in Acrobat. I'd recommend clearing the virtual printer output folder between batches.
