What's the recommended level to output from the MASTER OUT terminal without distortion? Can I adjust the output?

If the MASTER OUT [ATT.] is set to 0 dB, output won't be distorted with the volume up to +25 dBu for the MASTER output (XLR).

Adjust the [ATT.] to set the attenuation level of audio output from the [MASTER] terminal. When the [PEAK LIMITER] is enabled, you'll be able to alleviate sudden, unpleasant digital clipping of the master output.

For details, see [Changing the settings] in [Settings] in the Instruction Manual. You can read the Instruction Manual here.