Protecting a PDF with a password
Two protections exist inside a PDF and they answer two different questions. This page says which one to ask for, and what neither of them can do.
Which of the two to ask for
The question to settle first is whether the document itself is the secret, or whether it is meant to be read and you would rather it were not lifted wholesale.
A password before the document opens at all
Nothing of the page is shown until the password is typed. Ask for this when the document itself is what has to stay private: a payslip, a medical result, a bank statement, a contract before it is signed.
It costs the sending of the password. It has to reach the reader by some route other than the document — a telephone call, a message, a channel already agreed — and a password sent in the same email as the file protects nothing at all.
No password to open, but printing and copying narrowed
The document opens for anybody, and what may be done with it once open is narrowed: printing, copying the text, changing the page. Ask for this when the document is meant to be read widely — a price list, a report, a specification — and you would rather it were not lifted into somebody else's document wholesale.
It costs nothing on the reader's side: there is nothing to type and nothing to send. What it buys is a statement of intent that the programs people actually use honour.
What a password does not do
Worth reading before it is promised to anybody, because all three surprise people. None of them is a fault of the format: they follow from what a password is.
- It does not stop a photograph of the screen. Whoever can read the page can point a telephone at it, and no document can prevent that.
- It does not stop the document being saved again without it. A program that has opened the document — with the password, or because none was needed — can write a copy that asks for nothing.
- It does not say who wrote the document, nor that nothing in it changed afterwards. A password answers who may open; only an electronic seal answers who wrote it and whether a single byte has moved since.
Where to go from here
Both documents below are published open, password and all, so the protection can be tried rather than taken on trust. Each of their pages explains its own protection in full and shows the program that writes it.
Where the question is proof rather than access — who signed, and whether the file has moved since — the page on the electronic seal is the one to read.
The electronic seal: who wrote the document, and whether it moved Everything else the engine puts on a page
Glossary
- byte
- The unit a file's weight is counted in, the way a parcel is counted in grams. A thousand of them make a kilobyte, and a million make a megabyte — the size of one photograph taken with a telephone. A one-page PDF here weighs between six thousand and a hundred and twenty thousand, so a hundred of them fit in the space of five photographs.