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 188.8.131.524, WinDjView 2.1 and Windows 10
A DJVU document typically contains both a layer of scanned image and a layer of the text in that image. Sometimes, a DJVU document is produced which does not have the text layer. This makes it hard to search and find text in the document.
Recognising the text in the DJVU document using OCR and adding that as a text layer to that document is easy:
DjVuLibre provides open source libraries, commandline tools and GUI tools for dealing with DJVU files. So all solutions for converting a DJVU file to a PDF file will use DjVuLibre is some form. Note that it looks like all conversion options seem to produce PDF by converting every DJVU page into an image. So, the resulting PDF files will be big and much larger than the original DJVU files. There seems to be no way to produce smaller PDF files.
Install DjVuLibre tools on Ubuntu:
$ sudo apt install djvulibre-bin
Convert using the ddjvu tool:
$ ddjvu -format=pdf in.djvu out.pdf
Tried with: Ubuntu 18.04
Install DjVuLibre and Ghostscript tools on Ubuntu:
Deleting one or more pages from a DjVu document can be done using the DjVu multipage document manipulation utility. This is the executable djvm.exe that ships with the DjVuLibre package.
The -d option can be used to delete a single page from the document. However, djvm does not accept multiple pages or a range of pages to delete. To achieve this, invoke djvm multiple times using the looping features of your shell. The example below is for PowerShell.