Hey prasoon123ak,
Do you own the PDF files or are they sent to you from another creator? If you own the files, then try the following steps:
1. In the Preview app on your Mac, open a PDF or image.
2. Move the pointer next to the document’s name in the title bar, click the arrow that appears, then select or deselect the Locked checkbox.
"To lock or unlock a file in the Finder, select the file, choose File > Get Info, then select or deselect the Locked checkbox."
This information can be found here: Lock PDFs and images in Preview on Mac
If the files are owned by another creator, then the creator must make these changes on their end.
We hope this helps!
Best regards.