If you want monochrome printing, then you’ll want thermal based technology, even if you don’t know it yet. These printers can only print in black and white, but are tiny in size being comparable to a laptop battery. Their resolution figures are on average quite low at around 200-300 dpi, but this is more than enough for simple non color prints.
What’s more is unlike inkjet printers they can store internal Bluetooth adapters so you don’t have to carry an adapter around with you. There generally designed for use with laptops but using an internet table or smart phone (as long as they have Bluetooth or infrared) to transfer the work wirelessly works perfectly.
The device you’ll be interested in the most will be the Brother PocketJet 6 Plus. In terms of specifications this is near the top of the range but without having a preposterous price tag. Now just remember that there are two versions of the same model, there’s the regular one which I’ve linked you above and then there’s the ‘with Bluetooth’ version. The regular one can still support Bluetooth but it’s an external adapter so I would recommend getting an internal one to begin with.
It’s definitely tiny and weighs just 1.3 pounds making it ultra lightweight too. One great feature is that the thermal paper is stored in rolls and housed within the main body. This means that you don’t have to be carrying around reams of paper with you whenever you go out. Furthermore it doesn’t use ink because of the thermal technology which means that overtime you’re going to save substantially (just consider what a rip off ink cartridges are).
I think you’ll find that for your purpose thermal portable printer is definitely the best option to go for. The downsides are the lack of color printing and low resolution figures, but for black and white you’ll still definitely be getting crisp black and white text. The PJ 6 Plus hits 300 dots per inch and its improvement over the regular PocketJet 6 is the 100 dots per inch increase. If you feel that you don’t need this then you can just get the regular version – and this too comes with the ‘with Bluetooth’ or regular version totaling 4 similar but very slightly different models.
If this isn’t what you’re looking for then I would suggest looking at some of Canon’s portable printers. The Canon iP100 is definitely the best out at the moment if you want an inkjet device, but just remember that you’re certainly going to be sacrificing the transportability of the product. This doesn’t mean it’s going to be massive but I’d liken it to a note book in size aside from a little smaller and twice as fat.