PDF Encryption with pdftk

You can encrypt any PDF created with pdftk by simply adding encryption parameters after the output filename, like so:

... output

\
[encrypt_40bit | encrypt_128bit] [allow ] \
[owner_pw ] [user_pw ]

Here are the details:

[encrypt_40bit | encrypt_128bit]
Specify an encryption strength. If this strength is not given along with other encryption parameters, it defaults to encrypt_128bit.

[allow ]
List the permissions to grant users. If this section is omitted, no permissions are granted. See Tables Table 5-1 and Table 5-2 for a complete list of available permissions.

[owner_pw ]
Use this combination to set the owner password. It can be omitted; in which case no owner password is set.

[user_pw ]
Use this parameter to set the user password. It can be omitted; in which case no user password is set.

Adding these parameters yields :

pdftk A=in1.pdf B=in2.pdf C=in3.pdf \
cat A1 B1-end C5 output out.pdf \
encrypt_128bit allow CopyContents Printing \
owner_pw ownpass