The Cloud:
If it is something you only do occasionally, and the files are not huge, the easiest way is to use a cloud service like Dropbox, Google Drive, iCloud Drive, or OneDrive. You can download the app for he service on the iPad, and/or connect to the service in the Files app.
You can add your third-party cloud services — like Box, Dropbox, OneDrive, Adobe Creative Cloud, Google Drive, and more — to the Files app so that you can access all of your files on all of your devices.
support.apple.com
You can find individual instructions for each service by searching Youtube, or just the web in general.
Apple's Solution:
It used to be that you could install and use iTunes on your Windows computer to do this. It looks like Apple has replaced that with program called Apple Devices. I've never used it, so can't say much more than that.
Copy files between your computer and apps on your iPhone or iPad.
support.apple.com
Connect to a Windows Server:
If you have smb (sharing) services set up on your Windows machine you can connect to it in the Files app on your iPad. Given the nature of most local networks the IP address of your Windows computer may occasionally change, so it can need to be redone, but in my experience it works well.
In Files on iPad, view files stored in iCloud Drive and on file servers, other cloud storage providers, USB drives, and SD cards.
support.apple.com
Email and/or Safari:
You can just email the file to yourself, and download it in the Mail app (or Safari if it's a link). Keep in mind that simply opening/viewing a file in Mail (or Safari) does not save it. You need to share/save it to the Files app, or another app when appropriate.
This is how I get most of my non-DRM books and documents, straight to the iPad from whatever link the site/email gives me. Honestly it's easier to just re-download stuff from the web than go through the trouble of transferring it from a Windows computer; or even my Mac (on the rare occasion it doesn't automatically sync)
Depending on what reader app(s) you are using, you'll probably need to share the file in Files to the app of your choice. Though some apps (like Documents by Readdle) have built in methods to transfer apps from your computer or the web. So check out your preferred read's features before making things more complicated than they might need to be.