This is my first time using a tablet and I'm totally lost - this must be what computers look like to old people.

This is a Lenovo Thinkpad tablet. I don't see a specific model number on it (it was passed down to me second hand).

What I'd like to do is read a PDF. I'm fairly certain I successfully transferred the PDF onto the tablet via an SD card, and I noticed this thing has an app called "eReader" on it, so I started that up.

All I see are empty bookshelves, and no obvious buttons or settings anywhere. I saw in multiple places that you have to, as one site puts it:

...copy and paste the relevant files into the eBooks folder and they will appear on the eReader shelf for your consumption...

But I have no idea how to:

1. Access the file system.
2. Locate the eBooks folder.
3. Copy and paste files.

Will I need a file explorer app? If so, how can I install that given that my only method of getting data onto this tablet is an SD card (no wireless internet).