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When I want a copy of an online receipt, I do a screen capture (press home and on simultaneously). An image is created in iPhoto, which can be emailed, copied, or printed just like any other file.
I do this with electronic boarding passes too. Easy to email to admin staff, and easy to keep on screen at check-in.
 
Have yet to find a single app that will print to pdf within the iOS world.

One always at least has be copy and paste a url address into some other program for that other program to create the pdf first.
 
So many options

There are so many options now. There are quite specifically three ways to accomplish this. You can use an app, like Instant PDF Converter
https://itunes.apple.com/us/app/instant-pdf-converter-+-email/id580089705?mt=8 which I use when I want to convert a single email at a time. Even Adobe has created an option. These apps usually range from near free, to rather expensive depending on all of the features you desire. As always, check the reviews, some of these apps are terrible.

You can use any free online service. There are probably close to 20 if you do a quick Google search. I use https://www.emailexporter.com/emailtopdf when I have a group of emails I want to convert as a batch.

Or you can go through the laborious method of converting to PDF on your computer and emailing it to yourself. ;) I've been there, done that, and it sucks.
 
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