PDF security is very important in business work. This article will tell you how to encrypt a PDF on Windows and Mac OS.

1. How to encrypt PDF files
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2. PDF is widely used in our daily work. It’s very important to learn
how to make a secure PDF as sometimes we don’t’ want it to be
seen by some other people. The general method of encrypting a
PDF is generate a password for it so that you must enter the valid
command every time you open it.
This article will tell you how to encrypt a PDF on Windows and Mac
OS. These two tricks are very easy (especially the Mac OS one).
Before we start, you should known two things:
1. All passwords have the possibility of being cracked
2. Don’t forget the passwords you set!

3. 1. Encrypt PDF on Windows
Visit www.pdfmate.com/setup_free_pdf_merger.exe to
download the PDFMate Free PDF Merger and install it on your
computer. Open the program and click “Add Files” to add PDF
files to the program.

4. At the “Output Setting” area of the interface, choose “Security”
and select both “Open Password” and “Permission Password”
options. Type any passwords you want. It’s better to set a longer
password as that would be more difficult to be cracked. Click
“Build” to generate a new PDF with password.

5. Open the new PDF file and you will meet a window to ask for
password. That means the PDF has encrypted successfully. Type
the password you just set before and you can read it again.

6. 2. Encrypt PDF on Mac OS
Encrypting PDF on Mac OS is very easy as the original PDF viewer
of Mac OS has the encryption feature. Open the PDF file with
“Preview” and choose “File” – “Export” at the top of the window.
Select “encrypt” and type a password, then click “OK” to save the
PDF.

7. Open the new generated PDF file and you can meet a similar
window to require the password.
Source: http://www.epubor.com/how-to-encrypt-pdf-files.html