I would like to share with you that you may not set different font properties in same paragraph while adding text to it. However, you may change any font properties once the text is added to PDF document. You can search for any word or set of words and then change any properties as per your requirements. Please visit below help topics for your kind reference.
.NET Article: [Search and Get Text from Pages of a PDF Document](("[[BL]]https://docs.aspose.com/pdf/net/search-and-get-text-from-all-pages-of-a-pdf-document/[[/BL]])
Java Article: Search and Get Text from Pages of a PDF Document
I hope this will be helpful. Please let us know if you need any further assistance.