A contract is sent by email to a client, an HR file to an external provider, a bank statement to their accountant. The PDF is sent as is, without protection. If someone intercepts the file or accesses the email inbox, the content is readable in plain text. Encrypting a PDF file before sharing it eliminates this risk at its root, provided the right method is chosen and one does not settle for a cosmetic password.
Metadata and hidden fields: what the password does not protect
Even before discussing encryption, an often-overlooked step is to consider. A PDF can contain metadata (author, software used, creation date), comments, hidden revisions, or invisible attachments. Adding a password does not erase this information.
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On a contract or legal file, this residual data can reveal names, previous versions of the document, or even deleted passages still present in the file. Cleaning the PDF before encrypting it is a precaution that most online guides overlook.
In Adobe Acrobat, the “Remove Hidden Information” function analyzes the file and lists everything that could leak: metadata, hidden layers, text under black rectangles. This is a real blind spot, because placing a black rectangle over a sensitive passage does not remove the text underneath. Anyone can copy-paste the “hidden” content. The actual editing permanently removes the text, unlike simple visual masking.
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If one searches for how to easily encrypt a PDF file, they mostly find tutorials on adding a password. However, the prior cleaning of the document is the first step in serious protection.
Open password or permissions password: two distinct levels of PDF protection
We often speak of “the” password of a PDF as if there were only one. In practice, two types coexist and do not protect the same thing.
- The open password prevents anyone from reading the file without entering it. The content is encrypted: without this password, the PDF remains unreadable.
- The permissions password restricts certain actions (printing, copying text, editing) but does not block reading. The document opens normally, only the functions are limited.
- Both can be combined on the same file. One then defines a password to open the document and a second to control what the recipient can do with it.
The distinction has a direct impact on the ground. A permissions password alone does not actually encrypt the content. Some third-party tools can bypass it easily. For a confidential document (contract, personal data, financial statement), the open password is the only one that guarantees true encryption of the file.
AES-256 encryption: the standard to aim for your PDF files
Not all PDF encryption tools are equal in the level of encryption applied. There are still online services that use older algorithms or AES-128 encryption. The latter remains solid, but when the option exists, AES-256 is the recommended standard for sensitive documents.
AES-256 is used by banking institutions and government agencies to protect their data. The principle: the content of the file is transformed into unreadable data using a 256-bit key. Without the password that allows reconstructing this key, the file is unusable.
Where to find AES-256 encryption in practice
Adobe Acrobat Pro offers AES-256 encryption in its security settings. When adding a password, one selects the compatibility level: choosing “Acrobat X and later” automatically activates AES-256.
For Mac users, the Preview application allows adding a password to a PDF via “File > Export as PDF > Encrypt”. The level of encryption depends on the version of macOS, but recent versions use a robust algorithm.
Online tools like Smallpdf apply AES-128 encryption. This is sufficient for a regular document, but for a highly sensitive file, locally installed software offers more control over the level of protection.
Encrypting a PDF in Word or LibreOffice: an unknown alternative
One does not always think about it, but it is possible to protect a PDF right from the export from a word processor. In Microsoft Word, when saving as PDF (“File > Save As > PDF”), the option “Encrypt the document with a password” is accessible in the advanced settings.
LibreOffice Writer offers a similar feature: when exporting to PDF, a “Encrypt” checkbox allows setting an open password and a permissions password separately. This method avoids installing an additional tool when producing the document oneself.
Feedback varies on this point: some users find that the permissions password set via LibreOffice is less well respected by all PDF readers. For internal use or a one-time send, the method remains reliable. For a legal or regulatory document, one would prefer Acrobat or a dedicated encryption tool.
Best practices for an effective PDF password
AES-256 encryption is useless if the password is “123456” or the company name. A few concrete rules make the difference.
- Minimum 12 characters, with uppercase letters, lowercase letters, numbers, and special characters. A short password can be cracked in a few hours with current tools.
- Never send the password through the same channel as the file. If the PDF is sent by email, the password should be transmitted via SMS, encrypted messaging, or phone call.
- Use a unique password per document. Reusing the same password for all PDFs is like having only one lock for all your premises.
- Store the password in a dedicated manager (Bitwarden, KeePass) rather than in a text file or on a sticky note.
Protecting a PDF does not rely on a single action. It is the combination of cleaning metadata, the right type of password, solid encryption, and separate transmission of the access code that makes a document truly secure. Every link counts, and the weakest one determines the actual level of security of the file.