PDF Password Lock
Add password protection to your PDF files.
Browser Encryption Limitation
True PDF RC4/AES encryption requires native libraries not available in a browser sandbox. This tool re-serializes and tags the PDF: for strong password protection, use Adobe Acrobat, LibreOffice, or a desktop PDF tool.
Drop a PDF here or click to browse
PDF only · Single file
How to password protect a PDF file online for free
AlteredIdea's PDF Password Lock allows you to process a PDF document with password metadata entirely in your browser. Because no files are uploaded to a server, your sensitive documents remain completely private throughout the process.
It is important to understand a browser limitation: true PDF encryption (RC4 or AES-256, as used by Adobe Acrobat) requires native cryptography libraries that are not available in a browser sandbox. For documents requiring cryptographically strong password protection: such as legal contracts or financial files: we recommend using LibreOffice, Apple Preview, or Adobe Acrobat after preparing your document here.
How to add a password to a PDF: step by step
- 1Upload your PDF
Drop the PDF onto the upload zone. It stays in your browser: nothing is sent to a server. - 2Enter a user password
Type the password you want to associate with the document. This is required. - 3Enter an owner password (optional)
Add a permissions password if you also want to restrict editing or printing rights. - 4Click Process PDF
The PDF is re-serialized with your password metadata applied locally. - 5Download and apply strong encryption
Download the processed file. For cryptographic locking, open it in LibreOffice or Adobe Acrobat and apply AES encryption there.
Related Tools
Frequently Asked Questions
- Can I password protect a PDF for free?
- Yes. AlteredIdea's PDF Password Lock processes your file in your browser and applies password metadata to your document at no cost.
- Why does this tool note encryption limitations?
- True PDF RC4/AES encryption requires native cryptography libraries that are not available in browser sandboxes. For strong password protection, we recommend LibreOffice, Adobe Acrobat, or a dedicated desktop PDF tool.
- What does the tool actually do to the PDF?
- It re-serializes the PDF structure and sets password metadata in the document info fields. This marks the document with your password context but does not apply cryptographic locking.
- Are my files uploaded to process the password?
- No. All processing happens locally in your browser. Your document never leaves your device.
- What is the difference between a user password and owner password?
- A user password restricts who can open the document. An owner password (permissions password) controls editing, printing, and copying rights. Both are standard PDF concepts.
- How do I add a password to a PDF on a Mac for free?
- Use AlteredIdea in your browser, or use the macOS built-in Preview app: open the PDF, go to File > Export as PDF, check Encrypt, and set a password.
- How do I remove a password from a PDF?
- If you have the password, open the PDF in any PDF viewer, then re-save or print to PDF without encryption enabled.
- Is my password sent to any server?
- No. The password is only used locally in your browser and is never transmitted anywhere.
- Does the tool work on mobile?
- Yes. It works in any modern mobile browser on Android or iOS.
- What PDF viewers support password-protected PDFs?
- All major PDF viewers including Adobe Reader, PDF Expert, Preview on Mac, and browser-embedded viewers support password-protected PDFs.
AlteredIdea vs alternatives
vs Smallpdf / ILovePDF: Those services upload your files to their servers. AlteredIdea processes everything in your browser: your files never leave your device.
vs Adobe Acrobat online: Adobe requires an account and has file size limits on the free tier. AlteredIdea is unlimited and account-free.
vs desktop software: No installation, no licence fees, no OS dependency. Works on any device with a browser.