In the end I went with PDF attachments as per the mtone's suggestion in a comment on Alan G's answer. Here's how to add an attachment to a PDF (quick summary: in Acrobat X Pro, it's just a case of clicking the paperclip icon on the right, and then adding the file from that window and saving).
Here's the results of my basic testing, comparing the two options:
Attachments
- In Acrobat and Reader
- Opens showing the PDF in the normal way, with the 'Attachments' panel to the left of the page open by default, showing the attachment (very small) with a file-type specific icon (also very small). The filename is cut very short, so to be obvious to average users what's going on, a very short self-explanatory file name (e.g. "DATA.xls") is a good idea.
- In non-Adobe (e.g. Chrome, Mac Preview)
- Opens the PDF as normal. There's no evidence that there is an attachment nor any obvious way to access it if a user was expecting it. That said, there's also no harm done compared with having no attachment, other than increased file size.
PDF Portfolios
- In Acrobat and Reader
- A very different interface is shown. You've got a few choices of (in my opinion rather cheesy) 2006-style "wooshy" interfaces for clicking through the contents of the portfolio, where each item needs to be double-clicked on to be seen in anything other than a preview. They're all pretty distracting and not very intuitive to someone expecting a normal PDF. The main PDF's first page is previewed in the centre. If it's more than one page, an average user might not realise that they can get more pages by double-clicking to open it - people familiar with 'PDF page turner' interfaces might think that the things they see are all the pages they can get.
- In older versions of Reader / Acrobat (7 and earlier)
- According to this thread, portfolios are treated as PDFs with attachments, but a "bullish" message is displayed asking the user to upgrade. There's discussion of many cases where this is undesirable and causes wasted time and distraction.
- In non-Adobe (e.g. Chrome, Mac Preview)
- A big ugly message telling the user to get Adobe Reader, and no way I can see for the user to just see the original PDF if this isn't an option (e.g. if they're on a controlled network, or, if they don't want to install ~500 MBs (!!) of software just to look at one PDF...).
In my case, attachments are definitely the better way to go - our PDFs are deliberately simple (for home consumption not professional printing) so cross-client compatibility is important and not giving more work or annoyance to our end users is very important. Also, the key 'feature' portfolios have over attachments (a swooshy interface) feels in our case more like an obtrusion than a benefit.
If you go down the portfolio road, check Matthew Oglethorpe's comment in the above linked thread on PDF Portfolio backward compatibility. Apparently, if you replace the Cover page for the portfolio in the "vertical Icon bar (left column, 3 buttons on my set up), click Pages icon(topmost)" under View > Portfolio > Cover page, you can remove the nag screen for people in older versions of Acrobat. Might only work in Acrobat 9, not tested.
Looking at the way that, on a Mac, this ugly nag screen is used as the file icon on the desktop, I think the above method might also work for replacing the nag screen for non-Adobe users with something useful. Again, not tested.