Some PDFs should not be readable by just anyone who gets hold of them: salary slips, bank statements, medical reports, signed contracts, scans of your passport. Email gets forwarded, phones get lost and shared drives are rarely as private as they seem. Protect PDF locks your file with a password, so it can only be opened by someone who knows it.
Type a password twice, to be sure there is no typo, and save. Your PDF is encrypted with AES-256, the strongest encryption the PDF format offers and the same kind of protection used by banks. Anyone who opens the file in any PDF reader, on a phone or computer, is asked for the password first. Without it, the content cannot be read. Use Show if you want to check what you typed.
You can also decide what people may do once the file is open. Printing is allowed by default, because most recipients need to print. Copying text and editing are blocked by default, which discourages people from lifting parts of your document. These permissions are respected by mainstream PDF readers, but they are a polite rule rather than a strong lock: the password to open the file is what really protects it.
Choose a password that is long and hard to guess, ideally a few unrelated words, and send it through a different channel than the file itself, for example the file by email and the password by phone. Keep it somewhere safe. Nobody can recover a forgotten password, not even us, so keep an unlocked copy of important originals.
Encryption happens entirely in your browser. Neither the file nor the password is ever sent to a server, which is exactly what you want from a tool that protects private documents. If you later need to remove the password from a file you own, use Unlock PDF. A file that already has a password must be unlocked before you can protect it with a new one.