Yes, you can apply same Font-Family to entire document. Please see below sample code snippet for your reference.
[Java]
com.aspose.pdf.Document pdf1 = new com.aspose.pdf.Document();
com.aspose.pdf.TextState textState = new com.aspose.pdf.TextState();
textState.setForegroundColor(com.aspose.pdf.Color.getGreen());
textState.setFont(FontRepository.findFont("Times New Roman"));
pdf1.getPageInfo().setDefaultTextState(textState);
pdf1.getPages().add();
pdf1.getPages().get_Item(1).getParagraphs().add(new com.aspose.pdf.TextFragment("Hello World in Times New Roman"));
pdf1.getPages().get_Item(1).getParagraphs().add(new com.aspose.pdf.TextFragment("Second Text line..."));
pdf1.save("c:/pdftest/outputresults.pdf");
pdf1.close();
If you still face any issue, please feel free to contact us.
I am afraid, there is no specific approach to define font name for whole existing PDF document but you need to explicitly define the font information for individual TextFragment and TextSegment. The following code snippets shows how to replace font inside PDF document. It traverse through all TextSegments inside PDF file and then update the Font information.