pdf conversion for mobile devices

I am developing an android application to read a pdf file downloaded from a remote service.

 PdfRenderer renderer = new PdfRenderer(getSeekableFileDescriptor());
 final int pageCount = renderer.getPageCount();
 for (int i = 0; i < pageCount; i++) {
     Page page = renderer.openPage(i);
     page.close();
 }
 renderer.close();

The pdf file size range from 5MB to 50MB. I wanted to reduce the file-size of the pdf and I have tried the below suggestion which has reduced the pdf file size but not significantly.

• https://askubuntu.com/questions/113544/how-can-i-reduce-the-file-size-of-a-scanned-pdf-file

I also tried extracting each page of the pdf as image and then reducing the image width/height quality and then converting it back to pdf but that also did not help as the extracted total images were of very high size.

Other attempt I tried was to use calibre to generate epub from it but the generated epub does not have all the content from the pdf.

Any approach suggestion of how to compress the file size for mobile app?