A PDF is a static e-book format with fixed pages and a layout readable on PCs, tablets, smartphones, and most e-readers. You can therefore quote from it just as you would from a printed book.
With the EPUB, the text adapts dynamically to the reading device (e-reader) as soon as the font size or the font itsfelf changes. EPUBs are displayed by all standard e-readers (except the Amazon Kindle family), e.g. Tolino, Kobo, Sony Reader. The format is also particularly suitable for smaller devices such as smartphones.
The MOBI format was a special e-book format for the Amazon Kindle devices. Like the EPUB, it dynamically adapted to the reader, but is now obsolete. By the end of 2022 at the latest, Amazon will also switch completely to the EPUB format.
There are also numerous programs that allow reading EPUB and MOBI files on a PC or tablet (e.g. Adobe Digital Edition, Kindle app).