Do not grab paper until printing is complete.Grab is a word you don't see written down that often. It has a very specific meaning, involving a hasty movement and perhaps negative connotations. Normally, you're being told not to grab (usually if you're a child), or to grab something quickly with the implication that it's a slightly naughty thing to do. Imagine you're at a buffet, and there are chocolate brownies at the end of the table. You're still getting sandwiches, but you say I'm going to grab a brownie while they're still there. You shouldn't really, because you're effectively queue-jumping, so it's a very slight misdemeanour.
In this case, the verb is very appropriate. It is a bad thing to grab the paper too soon, as you will spoil the printing and possibly damage the printer. But also, grabbing is exactly what you would have to do. They could have said Do not take paper until printing is complete, but by using grab, a very clear image of quickly reaching in and trying to take a bit of moving paper is conjured up, and the repercussions of said action are also brought to mind.