What is best to protect floor of garden shed from chains

Several coats of oil based sealer would be one idea, but I would think that an epoxy sealer would be the best. You'll need to watch your temperature though. It may be too late in the season in PA to get any sealer to work it's best. Check the application temperature range on any product you choose before you put it on. If it's too cold, it will not cure properly and you could end up with a big mess. Wishing you the best on your project.

use an old floor mat............you need a new one at the front door anyhow

For now, until you can protect it further in the spring, put down something waterproof, like a rubber mat, then put an old rug on the top that will run the length the snowblower will travel in the shed.

Use a heavy rubber mat

i have used flat dish drainers.

Right, Louisx! A 3’x6’ piece of outdoor carpet, indoor carpet, linoleum, carpet pad, or even cardboard will be a good landing pad for any equipment with gas or oil, or chains that could gradually chip up your floor. And you should be able to get one or more of these at a carpet store or a home improvement store (ask for scraps). A cheap floor runner would work and they are widely available. And big pieces of cardboard come from a big cardboard box from your house or behind many stores. Don’t forget to get a new welcome mat for outside and inside of your house, at both doors. Best wishes 😇