PDFのパスワードを忘れた？選択肢について正直に解説

It happens to everyone eventually. You password-protected a PDF months ago, and now you can't remember the password. Or a colleague sent you a restricted PDF and left the company. Or your bank sends statements with passwords you never set up.

Let me save you some time: the answer depends entirely on which type of password your PDF uses.

The Two Types of PDF Passwords

PDFs support two completely different password systems, and people mix them up constantly.

Type 1: Permissions Password (Restrictions)

This is the most common type. The PDF opens normally — you can read every page — but certain actions are blocked: copying text, printing, editing, or extracting pages. You'll see this in Adobe Reader under File → Properties → Security, where it says things like "Printing: Not Allowed" or "Content Copying: Not Allowed."

Can you remove it? Yes. A permissions password is essentially an honor system — it tells PDF readers to restrict certain actions, but the underlying content isn't encrypted. The Unlock PDF tool can remove these restrictions.

This isn't a hack. The PDF specification itself acknowledges that permissions passwords are "best effort" restrictions that compliant readers enforce but don't cryptographically protect. Adobe says as much in their documentation.

Type 2: Open Password (Encryption)

This is the serious one. The PDF is encrypted — the actual file content is scrambled, and you need the password to decrypt it. Without the password, the file is literally unreadable. When you try to open it, your PDF reader asks for a password before showing anything.

Can you remove it? No. Not without the password, and not with any legitimate tool. The encryption (typically AES-128 or AES-256) is mathematically secure. If the password is long and random, no one is getting in.

I know there are websites claiming to "crack" PDF passwords. Here's the reality:

• Short, simple passwords (like "1234" or "password") can be brute-forced in seconds to minutes. Some tools try common passwords and short combinations.
• Medium passwords (8+ characters, mixed case and numbers) take hours to days on specialized hardware.
• Strong passwords (12+ characters, mixed case, numbers, symbols) are effectively uncrackable with current technology. We're talking centuries of computation.

If your encrypted PDF has a strong password, your only real options are:

1. Remember the password (check old emails, notes, password managers).
2. Contact whoever created the PDF and ask for the password or an unprotected copy.
3. Check if you have the original source file (Word doc, etc.) that the PDF was created from.

Common Scenarios and What to Do

"My bank sends password-protected statements"

Banks typically use your date of birth, account number, or a fixed password pattern. Check:

• The email that accompanied the statement — it often says "Password is your DOB in DDMMYYYY format" or similar.
• The bank's FAQ page for PDF statement passwords.
• Common patterns: DDMMYYYY, last 4 digits of account number, or your PAN/tax ID.

"I protected my own file and forgot the password"

If you set a permissions password (the file opens but you can't edit/print): use Unlock PDF.

If you set an open password: check your password manager, email drafts, and notes app. If you can't find it and you still have the original source file (Word doc, InDesign file, etc.), just re-export to PDF without a password. If you don't have the source and the password is truly lost, the content is effectively gone.

"A former colleague's PDF is locked"

If the file opens but has restrictions: remove them with Unlock PDF if you're authorized to access the content.

If it requires a password to open: contact the former colleague, or check if they stored the password in a shared password manager or internal documentation. IT departments sometimes have master passwords for company documents.

"I downloaded a PDF and it asks for a password"

Some publishers and government agencies protect downloads with generic passwords. Check the download page — the password is often listed below the download link, in the footer, or in the confirmation email. Common ones for government PDFs: "1234", the document number, or the publishing date.

If it's a pirated or unauthorized copy of a commercial document, that's why it's locked, and I'm not going to help with that.

Protecting Your PDFs Going Forward

A few tips to avoid the "forgot password" problem in the future:

Store PDF passwords in a password manager. 1Password, Bitwarden, and KeePass all support storing arbitrary notes. Create an entry for "PDF passwords" and log each one.

Use permissions passwords instead of open passwords when you only need to prevent casual copying or printing. Permissions passwords don't lock you out of reading the file — they just restrict what others can do with it.

Use the same strong password for groups of related files (e.g., all 2026 tax documents use the same password) and store that one password securely. Using unique passwords for every single PDF is a recipe for losing access.

Keep source files. If you have the Word doc or InDesign file that generated the PDF, you can always re-export without a password. This is your real insurance policy.

What About "PDF Password Remover" Software?

There are desktop applications that claim to remove PDF passwords. A few notes:

• For permissions passwords, they work — but so do free browser tools like Unlock PDF, so there's no need to install software.
• For open passwords, they attempt brute force or dictionary attacks. This works on weak passwords and fails on strong ones. Some of these programs are also bundled with malware — be extremely careful about what you download and install.
• The legitimate ones (like John the Ripper or hashcat, which are open-source security tools) require technical knowledge and are designed for security professionals, not casual users.

If your PDF has a strong open password, no software will help. This is by design — encryption exists specifically to prevent unauthorized access.