It is a fungal disease spots. I can't tell you what the responsible organism is, but guignardia/phyllosticta will be very likely as it is common in dendrobiums as well as other orchid genera.
Once an orchid gets this fungus, it is almost impossible to get rid of. You want to cut off the affected area, not the entire leaf, because leaving the spotted area encourages the fungal spore to spread.
Also, for future preventive measure, try not to mist the leaves.
Misting is not harmful but it makes it easier for any fungal spore that lands on your plants' leaves to germinate and affect the new host.
As long as you grow your orchids healthy, they (unaffected ones) should be able to fight off this disease on their own. Again,
misting aids fungus not orchids, especially so because you already have lots of fungal spores in the air in your home because of these spots on the leaves.
Cutting leaves off do not kill dendrobiums but it does reduce food production for the plant. So I recommend trimming the spotted area not the whole leaf, unless the entire leaf is spotted.
Make sure you use steril cutting tool for each plant.
Oh, I remembered something. If you want to opt not to cut the leaf, then you could use clear nail polish to cover the spots.
This way you seal up the fungal spots and there will be no more spores coming out of those spots.
This is something I read online.
Continue to provide your dendrobiums high light as doing so discourages fungal growth and keeps plants healthier.
Good luck!
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