There are several ways to convert DjVu file to PDF. All these methods result in PDF file sizes that are quite large, in hundreds of MBs. I have found that the converting using Foxit Reader results in the smallest PDF file sizes.
To do this:
Install the free Foxit Reader from here. This also installs a PDF Printer which we will use for conversion.
Install any DjVu viewer. I use WinDjView from here.
Open the DjVu file in WinDjView. Choose to print it. In the printer list, choose Foxit Reader PDF Printer. This printer has settings that you can modify. For the print settings, choose a page size (Letter) and remember to choose Scale to fit media. This will use the Foxit PDF Printer to write a PDF file for you.
Note that the settings of the Foxit Reader PDF Printer did not seem to have much effect on the file size. Default is 600 DPI. I did not see much reduction in filesize by using 300 or 200 DPI.
Note that the resulting PDF does not have OCR. You would need to run it through a OCR tool to get text embedded in the PDF.
Tried with: Foxit Reader 126.96.36.1994, WinDjView 2.1 and Windows 10