If you have an Office 365 account, move the Windows files to OneDrive. Make sure you are signed into the same account in Excel on the iPad, and you will be able to see and edit those files there as well. This is the most convinient solution if you want to keep the files up-to-date on both devices.
Other possible cloud solution include DropBox, Box, and a few others. Not quite as convinient as OndDrive, but still fairly easy.
iCloud Drive is usable, but poorly supported on Windows. It's better on a Mac, but not as well integrated with the Office apps as One Drive.
If it is only a few files, you can email them to yourself, tap hold the file/attachment in the email on the iPad for options to copy the file to Excel. Of course, files will not automatically update.
There are file-managment-like apps that you can use to transfer files between the iPad and any computer that has a web browser (when on the same network). It's doable, but always a two step process, because you must pass all files through the file app; copy to/from the computer then to/from the Office app. I've used GoodReader to do this, though it's not specifically designed for this task.
Documents by Readle also has a wi-fi drive feature that works through a web browser. You can view, but not edit, most Word formats in the Document's app. It also acts as a Docuemtn Provider, so you can bypass the two-step copy processes mentioned above. You can open files directly in Document's folders. I'm pretty sure, but not certain, that you need iCloud Drive on to do this.
You can also use the iTunes File Sharing feature of iTunes on the comptuer. It's inconvineint, but works. Again, files won't update accross devices.
About File Sharing on iPhone, iPad, and iPod touch
As far as the document format is concerned, the Office documents will ask you if want to convert to the most recent format when opened. As far as I can tell, you have to do this to edit the documents; but not view them.
I don't use the Office apps a lot, so I may be wrong with a few of these details. I did some quick checks as I wrote the post, but didn't dig into the details.