A Bitcoin paper wallet contains a pair keys: public key (address used to receive coins) and private key (allows to spend coins stored at that address).
Those keys are printed in a form of QR-codes so that everybody can quickly scan them and add the keys to a software wallet to make a transaction.
Now, if I lose my Bitcoin paper wallet, someone who finds it, is he able to spend my coins by scanning QR-codes ? And if yes, is this type of wallet secure ?
Yes, if someone locates the paper wallet and is able to read the key on it, they can spend any BTC sent to that wallet.
The wallet is secure if used correctly. Paper wallets are usually intended to be stored in a safe place, such as a literal safe or bank vault. They are not meant to be left on desks, dropped on the street, etc.
Additionally, you could utilize BIP38 to encrypt the private key on the wallet with a passphrase, that only you know.
Remember, any system is only as secure as its weakest link. Even a state of the art hardware wallet is useless if your seed word backup is easily accessible by an attacker, since they can recover the entire wallet from that.
