@RushikeshKondke
If you planning to work with Aspose Pdf you will require to buy license for Aspose Pdf .
But it seems that solution proposed by alexey.noskov to use Aspose Words to encrypt PDF is more reasonable one for your problem.
Unfortunately PDF File Processor license isn’t related to Aspose Pdf products scope and I couldn’t find documentation for it so I can’t tell if it’s possible to use it for document encryption