Hi Gal,
To try and make this a little simpler.
When you connect the Kindle to your PC, it is available as a standard windows drive letter. As you can see in
this link. Which shows how to look at it in Windows file explorer.
The problem is a kindle, any kindle, sucks for reading in Acrobat format (PDF files). It likes their own format best, AZW, but supports others.
So if you have, say, a MOBI file you can just copy it with Windows and it will appear.
However most ebooks are ePub or PDF. In this case you need to convert it. The tool I mainly use for conversions is Calibre. It is a bit busy but when you get used to the menu, you can import the book you just downloaded and simply tell it to send the book to the Kindle as Mobi. Job done.
To use Calibre to send files to your paperwhite, there is a tutorial
here.
Just one point which is irksome. You don't have to send via email. Just click next on the page which asks for your email details and it will complete (I tested that). The rest of the info is the same. Your Kindle should become a device on the icon bar on the top and you should be able to see it and all the books on it.
Once this is set up, you can add and remove books from your kindle simply. So long as you pick the paperwhite from the startup menu Calibre has all the info required to send the book for you.
There is a
video about adding books to Calibre and then sending them to your device (once you have it set up and plugged in). Don't be confused with the look and feel, the person is using Linux, which is a bit odd but it is recognisably the same in windows.
It doesn't look too technical to me, but you know I'm very technical.
Let us know if you want to try.