How much can a stepper motor take?

I'm converting an old inkjet printer into a plotter. It has two stepper motors, which according to their lable work with 7V. I have Sparkfun's EasyDriver, which I connect directly to the bipolar stepper motor. Is 7 V the highest voltage I can use? Or is it ok to use say 12 V, if I just adjust the current with the pot on the EasyDriver?

For instance, I have a stepper motor marked as 4 volts, BUT the test set I disassembled used 40 volt power supplies for the motors. They tested a commercial device that pumped dry wall mud and fed out corner papers with the mud on both sides.
So the motors produced a lot of torque. If you are not needing a lot of torque which will overheat the motors, you should be ok with 12 volts.

The real limiting factor is stepper motor temperature. If you have a motor that has a spec sheet that shows maximum operating temperature, it can be surprisingly hot: enough to burn your hand if you touch it.

Failing that, use the motor's maximum specified current which is usually available. As long as you don't exceed that, you can crank the voltage up. Higher voltage allows the motor to move smoothly at faster speeds. If you don't need the speed, keep the voltage low to reduce power consumption.

Well, the load is not big, I guess. One motor moves the paper through a reducing gear. The other motor moves the printer head back and forth. So I thought of starting at low current and seek for the smooth movement. I have no idea whether the motors can perform micro stepping. I forgot to read everything on the lable on the motor while I had the printer in parts. I only read the 7 V part. Right now I have only the wires coming out from the printer. I guess I need to open it one more time.

This is some specs I found:

Nominal Voltage 7
Current 0.7
Resistance (Ohms) 10
Torque (in-oz) 44.4
Torque (kg*cm) 3.2
Wires 4
Condition pulled
Steps / Revolution 200
Step Size (degrees) 1.8
NEMA frame size 17

...on this page:

Steppers are current driven and normally the voltage is irrelevant, so long as a current-controlling stepper driver is used. For the EasyDriver set the current no higher than the stepper's current rating (0.7A) and all will be well if you power it from 12V or 24V or whatever.

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The EasyDriver can provide some 50 mA from the 5 V pin. I guess this could be enough for the Arduino. I hope to have as simple as possible power solution for my project. Just one power source, 12 V or 24 V providing some 2-3 amps. This power goes into the EasyDriver, which in turn provides the 5 V to the Arduino. The second EasyDriver will power three LEDs, which will sink to pins on the Arduino.
The EasyDriver documents says:

It can take a maximum motor drive voltage of around 30V, and includes on-board 5V regulation, so only one supply is necessary.

It also says:

5V : This is an OUTPUT pin that will provide either 5V (default) or 3.3V from the voltage regulator, at a small amount of current (say 50mA - depends on input voltage) to power a circuit that you may need powered.

So at its max, the regulator would take down 25 V at 50 mA. That's 1.25 W. In my case, if I used a 19 V power source (a HP laptop power source) and my Arduino would draw 25 mA (just running code, no current draw from pins), I'd burn off maybe 0.35 W in the regulator. That should be ok, right?

My observation: Stepper motor torque is dependent upon current. The coils have a current ramp-up time which can be shortened by increasing the voltage. Hence the higher the voltage, the better the torque - even with the same max current.
if you user a driver with a current limiter, then I have found in my projects that a higher voltage is usually OK, and I get good torque.

Others are correct - things can get hot so be aware of that.
However, it depends if you need the stepper motor to hold a static position (using its holding torque). If you simply want to move the motor then stop without needing its holding torque, then most drivers have a disable pin, which removes the power from the motor. And that helps with heat.