PDF Security & Signature

Protect and sign PDFs online free — encrypt with password, decrypt, add watermarks, e-sign, fill forms, flatten. All processing in your browser. No file upload. Also: PDF Editor | Compress | OCR | Utilities.
Permissions (owner-restricted):
Encrypt: Sets a user password required to open the PDF. An optional owner password can restrict printing, modifying, copying, and annotating. Leave owner password empty to enforce restrictions with just the user password.
Decrypt: Removes the password protection from a PDF. You need to provide the correct password. The decrypted PDF will have no password.
Unlock: Removes owner restrictions (print/copy/edit limits) from a PDF. If you know the owner password, enter it. Password-free PDFs can also be unlocked.
E-Sign: Embeds a visual signature image onto a PDF page. This is a visual signature only (not a cryptographic digital certificate). For legal e-signatures, use a dedicated e-signature service.
Fill Form: Loads a PDF with fillable form fields (AcroForm). Edit each field value in the list below, then save the filled PDF. Fields are not flattened by default.
Watermark: Overlays text or an image onto each page. Text watermarks can be customized with color, opacity, rotation. Image watermarks are scaled and centered. Use diagonal text for draft/confidential stamps.
Flatten: Converts all fillable form fields into permanent page content. After flattening, fields can no longer be edited but will display correctly in all PDF viewers. Good for finalizing forms before sharing.

What PDF password protection actually does

PDF encryption is built around two different passwords that do very different jobs. The user password (open password) is the real lock: without it, the file's strings and streams stay encrypted and the document cannot be opened at all. The owner password (permissions password) is not a lock on opening — it controls a set of permission flags: may the reader print, modify, copy text, or annotate. If you leave the owner password empty above, it defaults to your user password.

The key fact: the user password encrypts the content. The permission checkboxes are just flags that well-behaved PDF viewers agree to honor — they are a policy, not a cryptographic boundary.

RC4, AES and what this tool uses

PDF's standard security handler has gone through three eras. The original algorithm was RC4 with 40-bit, later 128-bit keys; this is the format this page's Encrypt tool produces (RC4, 128-bit key, revision 3 of the standard handler), and it opens in essentially every PDF viewer ever made. AES-128 arrived with Acrobat 7 (PDF 1.6), and AES-256 with Acrobat 9 and PDF 2.0 (ISO 32000-2). Newer is stronger and is what current versions of Acrobat write, but older viewers may not open AES-256 files — RC4's compatibility is still its main argument. For files that genuinely need confidentiality, the strength of your user password matters far more than which era of algorithm encrypts it.

Why Unlock works without the owner password

If a PDF has an owner password but no user password, its content is encrypted with an empty user password — anyone can open it, and only the permission flags are "protected". That is why the Unlock tool can strip print/copy/edit restrictions from such files without knowing anything. It also explains the division of labor here: Decrypt needs your user password because it must actually decrypt and rewrite the file; Unlock only needs to clear the flags.

Visual signature vs digital signature

The E-Sign tool places an image of a signature on the page — convenient for ceremony, but it proves nothing: anyone can paste the same picture. A cryptographic digital signature hashes the whole document and signs that hash with a certificate, so any later change to the file invalidates the signature. If you need provable authenticity or legal-grade e-signing, use a service that issues real digital signatures.

Common misconceptions

  • "The owner password stops people from copying text." It only asks compliant viewers to hide the copy option. The text is not separately protected — non-compliant tools can ignore the flags entirely.
  • "An owner-locked PDF is encrypted and confidential." If no user password is set, the file opens freely for anyone. Owner restrictions deter honest readers; they do not keep secrets.
  • "A pasted signature image makes the document authentic." An image is decoration. Only cryptographic signatures can detect tampering.
  • "Flattening protects the form." Flattening turns fields into ordinary page content so they display the same everywhere — it finalizes a form, it does not encrypt or lock anything.

Related tools: PDF structure explained (where encryption dictionaries live), PDF Edit & Organize, OCR & Recognition (searchable text from scans), Compress & Optimize.