Personally,Id stay away from mill sawdust. First off the chances of it containing mites and other pests is high. Second, storing piles of sawdust this is a risk of Spontaneous Combustion, but it has to be a fairly thick pile for that to happen. I like pine shavings myself, bought from the local co-op for about $6 its roughly a 2x3 foot bale that is compressed.
It would work fine. Sawdust has been used for bedding for various forms of livestock forever. I can't imagine why sawdust would contain fowl mites? If it's available to you at a reasonable cost go ahead & use it.
I used surplus sawdust from building our cabin as litter for the floor. my two reds were still young, and they tried bathing in it when i put it in the nest boxes, they broke eggs so I switched back to straw. Right now I have pine shavings in there, (floor and nest boxes) and they don't mind a bit.