I've had good luck with neem oil as a treatment for brown spots on plants. Better yet, take a leaf into your local nursery so that they can diagnose the problem and suggest the appropriate treatment. Good luck!

This plant is native to tropical rainforest (in western Africa). It is adaptive, but not comatose. It looks fine, and in dry, indoor air you will have to expect some crisping. It should be just fine.

The brown spots are symptoms of a physiological disorder called oedema/edema, which is almost always primarily driven by over-watering. BTW - It would have occurred before the plant came to be under your care. I'll leave something I wrote about oedema below.

Oedema

Oedema is a physiological
disorder that can affect all plants. It occurs when the plant takes
up more water than it can rid itself of via the process of
transpiration. The word itself means 'swelling', which is usually the
first symptom, and comes in the form of pale blisters or water-filled
bumps on foliage. Under a variety of circumstances/cultural
conditions, a plant's internal water pressure (turgidity) can become
so high that some leaf cells rupture and leak their contents into
inter-cellular spaces in leaf tissue, creating wet or weepy areas.
Symptoms vary by plant, but as the malady progresses, areas of the
leaf turn yellow, brown, brown with reddish overtones or even black,
with older damage appearing as corky/ scaly/ ridged patches, or
wart/gall-like bumpy growth. Symptoms are seen more frequently in
plants that are fleshy, are usually more pronounced on the underside
of leaves, and older/lower leaves are more likely to be affected than
younger/upper leaves.

Oedema is most common in
houseplants during the winter/early spring months, is driven
primarily by excessive water retention in the soil, and can be
intensified via several additional cultural influences. Cool
temperatures, high humidity levels, low light conditions, or partial
defoliation can individually or collectively act to intensify the
problem, as can anything else that slows transpiration. Nutritional
deficiencies of Ca and Mg are also known contributors to the malady.

Some things that can help you
prevent oedema:

* Increase light levels and
temperature

* Monitor water needs
carefully – avoid over-watering. I'd heartily recommend a soil with
drainage so sharp (fast) that when you to water to beyond the
saturation point you needn't worry about prolonged periods of soil
saturation wrecking root health/function. Your soil choice should be
a key that unlocks the solutions to many potential problems.

* Avoid misting or getting
water on foliage. It slows transpiration and increases turgidity.

* Water as soon as you get up
in the AM. When stomata close in preparation for the dark cycle,
turgidity builds. If you water early in the day, it gives the plant
an opportunity to remove (for its own needs) some of the excess water
in the soil.

* Put a fan in the room or otherwise increase air
flow/circulation. Avoid over-crowding your plants. Al