How can we lock pdf files and how to unlock it?

 How can we lock pdf files and how to unlock it?

Password protection of your PDF file ensures that the PDF can only be opened by the intended recipient of the file. Password protection can also be used to prevent users from printing or editing the document. These settings can be further tweaked to allow only low resolution printing, and varying degrees of editing.

METHOD 1
To password protect your PDF document in Adobe Acrobat Reader,
go to Advanced > Security > Encrypt With Password.
A prompt will appear asking if you are sure you want to change the security settings. Click on Yes. 

Select Encrypt all document contents, check Require password to open document, and enter the password in the “document open password” field.
If you want control on the permissions to print and edit the document, 
check Restrict editing and printing. Provide the password for changing these settings. Make sure Enable copying of text, images, and other content is unchecked. Click on OK.

A prompt will appear asking you to confirm the passwords you have entered. Another prompt states clearly that not all third-party programs respect the security settings in the PDF file. 

METHOD 2
To lock the pdf files, you need to set a password before you convert the file to pdf. It's easy to make it with Simpo PDF Creator. 

You can set "not allow printing", "not allow copy" etc.

To unlock the pdf, you could use password remover software, or convert the pdf to a new pdf again.

METHOD 3

If you want to lock your PDF file and then unlock it, you can try A-PDF Password Security. This software is professional on protecting any PDF file with different security levels, and you can set restrictions such as non-printing, non-copying to PDF files. And also you can use the tool to remove those restrictions as you doesn't need. 
METHOD 4

use Solid PDF Creator. This will allow you to print your files to PDF and you can set the 

security level that it can only be opened with a password you set. This is a free download and free trial

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