I've seen portable hard drives with built in wifi:
http://www.businessinsider.com/best-wifi-hard-drives-2011-8?op=1. OP could store the files on them and transfer them to either device.
I think I'm interpreting the original post as he currently downloads them into a smart phone (probably) before leaving a wifi-enabled area (maybe at work), then wants to work on or view some of them on an iPad. So why not store the files on one of these wifi drives and then access them with either iDevice instead? Yes, one more thing to carry but that seems a given per the requirements offered by OP.
Or maybe he's downloading them via cellular (away from wifi) to that phone and is then looking for a way to work on them on the iPad? In this case too, I think those wifi drives plus the right apps would be the way to move the files off the phone, onto the drive, then off the drive and onto the iPad.
For what it's worth OP, I jettisoned the cell phone (and the expensive monthly bill) for an iPad LTE. Paired with earbuds + mic and voip app, it is now my cell phone for those somewhat rare times I need to talk to someone. When I'm away from wifi and really need to connect, I can dynamically buy 1Gb for $20 via LTE but so far I haven't had to do that once (I always seem to find free wifi). However, even if my situation was more like yours- where I need access and can't always find free wifi- $20/month for one device that covers both needs is much less expensive than $100/month for 2 devices. iPad Mini with LTE + Earbuds with mic + Talkatone app & Google Voice & Skype & jiwire wifi finder apps make a great alternative to just giving AT&T or Verizon about $100/month forever. I get unlimited calling in and out, unlimited texting in and out for a data fee so far of $0/month (but sooner or later, I'll probably have to spend $20 for some LTE in some situation. I'm guessing that will be about 2 times per year.).