I recently purchased an armax 230 ISOTRAN Model 1521. I can’t find it on the web anywhere, and from what I understand it is an isolation transformer that takes in 115 VAC and outputs about 210 VAC. (I’ve plugged it in and it does this) I’m wondering if it is safe to “play” with this device and try to make my own adjustable DC power supply from it. I am currently a Junior in college studying electrical engineering, but I have no experience with high voltages yet. Any advice would be appreciated.
Isolation does not really mean "safe to play with" - it enhances safety under some conditions (reference to ground, IF the connected circuit is fully isolated from ground contact) and not others (reference to two points in the circuit supplied by the transformer.) If you obey the "one hand in your pocket rule" it helps increase your odds, as you hopefully only manage to contact one point in the circuit at that point. Beware of little things like providing a ground reference via your oscilloscope probe if you are depending on isolation to keep you whole.
While I snicker a little at calling 210VAC high voltage, (I spent some of my college years working in a lab where the charge voltage into the marx generators was 30-50KV before the marx was erected) it certainly is high enough to be potentially dangerous, which is probably what you mean (depending on how sweaty you are or whether you manage to pierce your skin with a wire or probe, somewhere around 48-60V is typically high enough to be dangerous (in the "lethal" sense) if it supplies 10 mA or more...)
Other than that, yes, well, it's a transformer, and with high (-ish) voltages you may want to take a bit more care about packaging your experimental setups so as not to inadvertently fry yourself - and/or others, pets, etc. If you decide to make a several-hundred-volt capable DC supply, think about things like (automatically) draining the filter capacitors before you open it up after it's been off for a while and get a big fat spark or a shock.
The transformer will be "safer" than the same voltage direct from mains as it will not be ground referenced.
Note that 200VAC + is lethally dangerous if you touch it. Even with dry hands and no earth if you make two point body contact to any body points across 200VAC+ you MAY die. I have had probably dozens of 230 VAC shocks across quite a long lifetime - and while some were quite nasty I was in all cases 'lucky'. For some people the very first is fatal.
Note that if you ground ANY one point of your isolated output circuit then the isolation effect is wholly lost.
What sort of power supplies do you have in mind that need 210 VAC? MUCH lower voltages are MUCH safer. Use 210VAC only if essential.