Intended audience and prerequisites
For those typing into search engines, the goal is usually to inspect the specific content. Here is what you can expect from the standard edition (e.g., 4th or 5th edition):
Before you click on a suspicious link offering a free PDF, check your university’s Springer or IEEE access. You likely already have the right to download the official version legally. If not, buy a used 4th edition for the price of a pizza. Your career in communications engineering is worth the investment.