The Best way to do this is, by creating your own WiFi Network, that means it will be free and you will not have to pay charge for transfering PDF files.
Step 1. Download The App “Files Lite”
Go official link here and download the free version. The free version allows a maximum limit to 200 MB
Step 2. Start the Application in your Iphone
Once you have downloaded & installed the application, start it in your ipod. Before that, be sure you are connected to a WiFi Network. When you start the app, it will prompt you an internet address i.e http://192.168.137.21:8080 or something like this. Open your favorite browser and go to this address by typing it in your address-bar.
If everything is all right, you will see a web-page, using which you can upload PDF files in your Ipod! Later, to read the PDFs, open the app in your Iphone and select the PDF you want to read. That’s so simple 😀