PDF stands for Portable Document Format
There is a website name "I love PDF" which can convert your PDFs into almost any format. Here's the link to the website, https://www.ilovepdf.com/. This is in my opinion the best platform for conversion of PDF in any desired format such as "JPG/JPEG", "DOC" etc.
This article is specifically on how to password lock your PDF.
1. Visit the website
2. Click on "All PDF Tools" and under the PDF Security section, click on "Protect PDF".
3. Then upload the PDF you want to password-protect.
4. After uploading, write down the password and hit on "Protect PDF".
5. Now your PDF has been password protected, just download the PDF and you are good to go.