For gratis solutions, you will get the best coverage from the following three applications, which can all convert many different file formats to PDF, and will work on Windows/Linux(/macos?).
LibreOffice : office files/text files
ImageMagick : image formats
I have come across another post on askubuntu that's also looking for a calibre alternative.
The accepted answer on that post basically suggests using calibres internal command line tool:
The Calibre install provides the command ebook-convert that will handle what you want, and there's no need to run Calibre.
ebook-convert file.pdf file.epub
The original ...
As from the question it seems that you have access to the original LaTeX information I would recommend converting from the LaTeX rather than the pdf format. This can be done with a number of tools, one of which is the python pylatexenc library which includes a LaTeX to text conversion tool that is usable both programmatically and from the command line.
I've noticed one of the resources you listed is the LEADTOOLS SDK. (Disclosure: I'm a LEADTOOLS employee).
Converting to the ePub file format from an input PDF can be done through the use of the DocumentConverter as you have listed https://www.leadtools.com/help/sdk/v21/dh/doxc/documentconverter.html
To test this in Android code, you can try the following: