OK, no need to finish paraphrasing the famous and somewhat off-color Mae West quote. Keyboards are large unweildy things that are definintely not pocketable - until now.
Recently iClever introduced a tri-fold Bluetooth keyboard that works with iOS, Android, Windows and Mac. It folds up small enough to fit into a pocket (assuming your pocket is fairly substantial. It certainly fits easily in any purse or briefcase. Bluetooth keyboards are handy for text input into a tablet or smart phone when using the voice to text dictation features would be inappropriate. You certainly can't dictate a confidential letter to a client using your voice while in a cafe, airport, or other public place.
The iClever keyboard sells for $35 on Amazon anc comes with a velour drawstring carry bag and a micro-USB charging cable. When you unfold it, it powers on ready to pair and connect to your phone or tablet. When you fold it up, the power turns off, saving the battery for your next use. I have not had mine long enough to use a full battery charge, but I suspect the keyboard will make it through even extended travel on a single charge. If not, it can be charged using the same cable as any Android phone, tablet, or a multitude of other devices using micro-USB charging.
The iClever does not have full-sized key spacing like you will find on a desktop keyboard. It is scaled down to about 85% of full size. Still, once you get used to the closer spacing, it is fairly easy for a touch typist like me to use. Key travel is actually very good for a portable folding keyboard.
I remain convinced that voice input is the best way to get text into your smart phone or tablet. Google's dictation service seems especially fast and accurate to me. But for those times when dictation would be inappropriate or could breach client confidentiality, a very compact Bluetooth keyboard like the iClever is a very handy accessory.