Draw it, type it, or photograph one you signed on paper. It waits here, encrypted on this device, ready to drop onto any PDF.
Your signature is the one genuinely sensitive thing this site touches, so this is spelled out rather than assumed. It is stored on this device and nowhere else. It is encrypted with a key derived from your PIN through 600,000 rounds of PBKDF2-SHA256 and sealed with AES-GCM; what actually sits in browser storage is a block of ciphertext with no trace of the picture in it. Opening it without the PIN yields nothing rather than something partial, and altering the stored block makes it refuse to open at all. The PIN is never stored, so nobody, us included, can recover it or read what it protects.
The limit, stated plainly: a short PIN is a short PIN. Someone who takes the stored block off your machine can try guesses against it at their leisure, and a handful of digits will not hold for long. Six or more characters with letters in them puts that out of reach. If you would rather keep nothing at all, you do not have to: the Edit tool will let you sign a document without ever saving the signature.