Three options are available to secure your PDF file,

Password Protection

You can set a Document Open Password for your PDF file to prevent unauthorized access. When this password is set, each time your PDF file is opened, a password prompt window pops up.

Change Permissions

You can secure your PDF file by setting a Permission Password to prohibit the following changes,

Inserting, deleting, and rotation of pages

Fill-in form fields and signing

Commenting, filling in form fields, and signing

Any except extracting pages

Copying of text, images, and other content

Text access for screen reader devices for the visually impaired.

Disallow Printing

You can also prevent your PDF file from printing by others by putting "None" in "Printing Allowed". If you would like to grant low resolution (150 dpi) printing permission, set "Low Resolution (150 dpi)" in "Printing Allowed". If you do not want to set restriction on printing, just set "High Resolution" in "Printing Allowed".