In the [unlikely] event I'd need to do that, I'd use my Mac, open in Excel, Print >> PDF.
That being said, you could either use an additional app from the App Store (I see a couple with this functionality), or use an online service (where you'd upload the file and download the PDF conversion).
You don't have to email... Send Attachment, PDF, Send a Copy and you can then even send to OneNote, Dropbox or Notes. Sort of strange that cannot save to OneDrive though.
What you can do is quickly send to Notes and then from there can do more with it like send copy to PDF Expert, GoodReader, iBooks etc. etc... Still would be nice to be able to do all this directly from Word or Excel though but can use Notes as an intermediate workaround.