DIAGNOSIS You're burning up the motor. This is caused by overloading—pushing too hard, using a dull blade, or cutting material that's too dense for the tool. The insulation on the motor windings is overheating.
FIX Stop using it immediately. Let it cool completely. The fix is behavioral. 1) Always use a sharp, appropriate blade for your material. A dull blade forces the motor to work way harder. 2) Let the saw do the work. Apply gentle forward pressure, but let the reciprocating action of the blade make the cut. If you're leaning on it, you're overloading it. 3) Make sure you're using a high-capacity ONE+ battery (4Ah or higher) for heavy cutting. A small battery can't deliver enough current and causes the motor to strain.
IF THAT DIDN'T WORK If the smell and smoke happened even under light use, the motor might already be damaged. The windings could be shorted. This requires a complete motor replacement, which is often as expensive as a new tool.
HEADS UP That smell is a serious warning. If you ignore it and keep running the saw, you will permanently destroy the motor. It's not a "break-in" smell. Ease up, use sharp blades, and take breaks during long cutting sessions.