You'd need to change the entire social structure and cultural outlook of the Roman civilisation for printing to have an effect other than a curiosity. There was simply no impetus for innovations to conserve manpower- labour was dirt cheap thanks to slavery and a (for the available technology) high population. Such advances are only put to use when there is a labour shortage (like in early medieval times, when the area of the former Roman Empire held more than 100 million people less than at its heyday, or in Late Medieval/Renaissance times, when plague and the beginning of the little ice age had shrunk Europes population.)
Second, the Empire was agrarian based, with far less emphasis on trade than in later time. Those 30% literates or so are about everyone who can make use of letters. A field worker doesn't need to read or write. Control over the written word was far more important to the Emperors than spreading it wider through the population. Someone brought the example of Puritan England stressing reading. Apart from the religious causes, which just aren't there in pagan Rome- English Puritanism also caused the civil war. Increased literacy also makes it easier to spread subversive ideas and weaken censorship. Not something the Empire might look favorable upon.
As for conquering northern Europe- what is there to conquer? The Empire was already hard to keep together. Not because of any seperatism- even the invading barbarians who brought Rome down in the end didn't originally want seperate Kingdoms- they just wanted to live in the Empire, with its luxuries and superior culture, preferentially as top dogs- but because usurpation was more or less the accepted method of succession, and there was almost always someone somewhere at least plotting to overthrow the Emperor militarily, if not actively campaigning. A bigger Empire would mean less central control over the distant provinces, making it easier for local strongmen to assemble armies.
On the other hand the Empire had reached its maximum borders. In the south the Sahara, in the north the woods of Germania and the Caledonian highlands, in the east Persia, which was too strong to conquer, barring the only route Rome had to expand. Rome's rule was based on the cities- or some equivalent, like the Gaulish oppida. Northern Europe didn't have cities, or even small towns. It also had abominable climate (from a Roman perspective), not much to loot save a few baubles and slaves, and the locals were hard to keep down.
This had something to do with the less stratified and more egalitarian social structure of the tribes beyond the Limes. Unlike the Celts, Italians, Greeks and Orientals, the Germans did not have an institution of absolute rule, either by a despot or the nobility as such. Varus failed because he didn't acknowledge that, couldn't acknowledge it, in fact.
Rome could only hold sway where there were population centers to keep down with their Legions, a local elite to bribe and coopt, and a mass of proles who didn't count politically. There was no way to deal with an independent-minded, dispersed population. Same problem the Brits had in the US war of independence. Same effect. They could pacify the tribes for a time with 'police actions' (or bribes), but they couldn't conquer them, all along the northern frontier from Germania's woods to the Skythian steppes.
