Welcome to the group Peto1
Why are the file download formats in .zip instead of .PDF or .epub?
Zip files are used to save space.
I find it difficult to unzip the downloaded file on my device
Not sure why you are having problems unzipping files. What unzip utility are you using? Every system comes with one, but you can always find one online. I use WinRAR and 7zip.
You said you are having problems unzipping files on your device. Are you using a mobile device, a pc, or a laptop?
Also,the sites I want to submit to insist on .epub format.
Sorry, but why do you need epub format? I hope you aren't trying to submit plr to sites like Kindle, where it's illegal and can get your account banned.