PDFをパスワードで保護 - 無料オンラインPDF暗号化

Protect PDF with Password — Encrypt PDF Online Free adds an open-password to a PDF so the document cannot be opened without the password you set. The tool accepts .pdf files only and wraps the file with standard PDF encryption (see encryption algorithm question below), it does not convert formats or edit page content. You can upload a single PDF or several PDFs (batch), click "Set Password" and then "Encrypt PDF" to produce a locked .pdf or a .zip containing locked PDFs. The resulting filename is the original name with "_protected.pdf" appended for single files, or a zip called "protected_pdfs.zip" for batches.

Open the page, click the "Upload PDF" button or drag & drop a .pdf into the drop area, then select your file from the Files dialog (desktop) or Files app (mobile). Enter the password into the "Password" field and repeat it in "Confirm password" so the two match, then click "Encrypt PDF"; a progress bar shows encryption progress and will finish in a few seconds for typical files. When ready click "Download" to save the locked PDF (browser will save to your default Downloads folder unless you choose otherwise). If you uploaded multiple files you will get a single ZIP download named "protected_pdfs.zip".

This tool accepts PDF files only (.pdf extension) and will reject non-PDF uploads. Single-file uploads are supported up to 100 MB per PDF; you can process up to 5 PDFs in one batch with a combined limit of about 300 MB. Those limits are realistic for Chrome and Firefox on desktop; mobile Safari and lower-memory browsers may struggle with files above ~30–50 MB and may fail to encrypt large batches. If your file is bigger than the limit, split the PDF in a desktop editor or use a desktop app like Acrobat.

The tool uses the browser's Web Crypto API to apply AES-256 encryption to the PDF open-password, which is the current standard for strong symmetric encryption in web apps. It sets an "open" password (required to open the document) rather than only owner permissions; the encrypted file uses standard PDF encryption wrappers so most PDF viewers will ask for the password. Because the work happens in-browser, the key derivation uses PBKDF2 with a salt and several thousand iterations to resist brute-force guessing. This offers good protection for reasonably strong passwords, but encryption strength depends on password complexity.

Encrypting a PDF with this tool does not recompress or alter page images, fonts, or vector content — it only wraps the existing PDF bytes in an encrypted payload, so visual quality remains identical. You may see a small size increase (typically 0.1–1%) due to encryption metadata and padding; if you upload a PDF that was already heavily compressed the resulting encrypted file will be nearly the same size. If you need to reduce file size as well, run a separate PDF optimization step in Acrobat or an online "Compress PDF" tool before encrypting to preserve the desired balance of quality and size.

If you get "File too large," the file likely exceeds the 100 MB per-file or 300 MB batch limit; split the PDF or use a desktop tool like Acrobat or qPDF for very large documents. "Upload failed" can result from unstable network connections or browser memory limits — retry in Chrome/Firefox desktop, or close other heavy tabs and re-upload. On iOS Safari backgrounding can abort large uploads; use a wired or strong Wi-Fi connection, or upload from a desktop. If the PDF is already password-protected or corrupted, the tool will display an "Unsupported PDF" error — open the file in a reader first to confirm it opens normally.

This online tool currently only sets an "open" (user) password that is required to open the PDF; it does not create a separate owner/permissions password to restrict printing, copying, or annotation. Most desktop apps like Adobe Acrobat Pro allow granular restrictions and dual passwords (user vs owner), which is preferable if you need permission-level controls. If you require permission locks, encrypt here for open-password protection and use Acrobat or other desktop software to set permissions, or use a PDF utility that explicitly supports owner passwords.

Yes — you can select up to 5 PDFs and encrypt them in a single batch. After encryption the tool bundles the locked PDFs into a zip file named "protected_pdfs.zip" which you download in one step; individual files inside the ZIP keep their original names with "_protected.pdf" appended. Batch processing is limited by total size (about 300 MB combined) and browser memory; for larger or very long batches use desktop batch tools in Acrobat or a command-line PDF library. If you prefer individual downloads, upload and encrypt files one at a time.

On Android Chrome and Firefox, uploading from local storage or cloud providers works reliably for files up to ~80–100 MB; the browser will prompt to save the resulting PDF to Downloads or to a cloud location. On iOS Safari, the Files app integration works but Safari often suspends background tabs and may fail with files bigger than ~20–30 MB, so use desktop for large PDFs or switch to a desktop browser. Some older browsers without full Web Crypto support may fall back or block encryption — if you see an error, update your browser to the latest Chrome, Firefox, or Edge for best results.

Encryption is performed in the browser using the Web Crypto API so the file contents and your password are not sent to an external server during the process; after you click "Encrypt PDF" the encrypted bytes are generated locally and then offered as a download. That said, network uploads can occur if you choose cloud storage via the browser's save dialog, and browser extensions could read data, so use a trusted browser and disable extensions if concerned. The site does not require signup and does not add watermarks; treat encryption as local but keep regular backups of original files.

Desktop tools like Adobe Acrobat Pro offer more features — dual user/owner passwords, fine-grained permission controls, batch automation and better handling of very large PDFs — and they encrypt offline without browser memory constraints. This web tool is free, requires no signup, and is convenient for single PDFs or small batches up to the stated limits, but it lacks owner-password permissions and enterprise integration. For legal or high-volume workflows use Acrobat, but for quick one-off password-protection without installing software this tool provides a fast browser-based option with AES-256 protection.

Use a strong, unique password of at least 12 characters combining letters, numbers, and symbols, and store it in a password manager (1Password/Bitwarden/LastPass) rather than relying on memory. The tool does not provide a password recovery or removal mechanism — if you forget the password the encrypted PDF cannot be opened except by brute-force or third-party recovery services, which are slow and unreliable. As a safeguard, keep an unencrypted backup in a secure location until you confirm the encrypted file opens on another device.