The short answer
Open the encryption tool, pick "encrypt", set an open password — thirty seconds, fully local. The encryption is AES-256, the strongest grade in the PDF standard, the same one banks use for statements. But two other things decide whether you are actually safe: the password itself must not be weak, and — critically — the file and its password must never travel the same channel. File on chat, password on the same chat, and the lock and key hang on the same door. That separation is the core habit this guide installs.
Which files deserve a lock
One criterion: could the holder do something with this file if it leaked? Copies of ID cards, photos of you holding your ID, bank card scans, employment contracts and resignation certificates, medical reports, forms with your address and phone — all yes, all encrypt. Lecture notes, brochures, recipes — leak-indifferent, skip the ceremony. High-frequency case: sending into a group chat — onboarding materials to an HR group, a medical report to an insurance agent. A group's forwarding reach is out of your control; with a lock, a forwarded file is just noise without the password.
"No editing" protects nothing
Many tools offer another checkbox: no password, just "disable printing" and "disable editing". Those permission flags are a gentleman's agreement inside the PDF standard — well-behaved readers honour them, and any other tool ignores them at will. The content was never encrypted; any software reads it directly. So anchor on this: only a password that is required to open the file is real protection. Permission flags can ride along, but treat them as decoration, not insurance.
The three-step routine
Step one: open the encryption tool, choose "encrypt", drop the file in. Step two: set the open password — the only required field, typed twice. Step three: encrypt and download. Everything happens in the browser; neither the file nor the password touches a server. The tool produces a new encrypted file and leaves the original untouched — keeping an unencrypted copy for yourself is fine, but move it out of your downloads and back into the archive folder so it cannot be attached by mistake.
How to set the password — and how to send it
Weak passwords make encryption theatre: birthdays, phone numbers, "123456", the filename itself — dictionary attacks clear them in a second. Workable strength is simple: eight-plus characters, letters mixed with digits — say, the last four digits of a reference number plus a phrase only you two know. The delivery matters more: file on chat, password by SMS, a phone call, or spoken in person. The rule in one line: whoever ends up with the file should not automatically hold the password too. Answering "what's the password" through a second channel is exactly the extra step that draws the security boundary.
Two things to clean on top of encryption
First, metadata: PDF properties often carry the author's name, the company, the creating software, even the folder path the file was first saved in — none of it encrypted, all of it travelling with the file. Strip those fields with the metadata editor before sending; on contracts and CVs an author field with your real name is pure downside. Second, hidden layers: scans sometimes carry an OCR text layer or comment layer — invisible on the page, fully extractable by copy-paste. The safe move is a send-version containing only the pages themselves: split out the bare pages, then encrypt.
Two classic failures and the recovery
Failure one: you forgot your own password. AES-256 has no recovery path — forgotten means permanently locked, so save the password into your password manager the moment you set it, or use a personal format you cannot forget. Failure two: file and password went out together and you noticed minutes later. Retract if you can; if not, do the real remediation — for replaceable credentials like ID photos, freeze and reissue as needed; for ordinary material, ask for deletion and resend a differently-passworded version. Remediation is always inferior to the one minute of locking beforehand.
想马上动手：PDF 加密 三十秒上锁，发送前用 元数据清理 抹掉作者信息，文件太大就先过一遍 PDF 压缩 再加密。Try it now: lock files in seconds with PDF encryption, strip author fields with the metadata editor, and slim big scans with the PDF compressor before encrypting.
FAQ
I received an encrypted PDF — can it be opened without the password?
AES-256 with an open password cannot be opened in any reasonable time frame without it. The "one-click unlock" tools online only work on legacy 40-bit RC4 encryption or files with permission flags but no open password. If it is your own file and you forgot the password, the legitimate route is asking the sender again; third-party crackers are both unreliable and off-limits on files that are not yours.
Can I compress or merge a PDF after encrypting it?
Order matters: compress, split and merge first; encrypt last. An encrypted file is ciphertext — compressing it yields nothing and most mergers refuse it outright. The standard pipeline is "strip metadata → compress/split → encrypt", with encryption as the final step before sending.
Does encryption make the file much larger?
Barely. AES-256 adds a small fixed overhead — kilobytes — negligible on a multi-megabyte file. Size is governed by the images inside the pages, so "too large" and "needs a password" are independent problems with independent fixes.