why is my libharu pdf oversized with .png images?

I am creating a pdf using libharu in C++ (compiled as a .cgi) that features .png images. The code is fine, but my pdf's are ridiculously oversized. Each page features one image of around 30kb and around 4 text characters in libharu's system font. If I open a 20 page output file of 25mb and "print" it to a file in my operating system it becomes 256kb or so with no visible change to the images.

I think the issue is related to libharu because this guy see's it too, here. He is using php so, libharu as a compiled .cgi. (my C++ code is also compiled .cgi, linked to libharu).

Another guy here on stack overflow has also seen size issues with libharu, but his problem does not mention anything to do with .png so it may be unrelated.

Code for reference:

WorkingGraphic = HPDF_LoadPngImageFromMem  (    *gPdfPtr,
                                                PngAssets[AssetIndex],  //Image data ptr
                                                PngSizes[AssetIndex]);  //data length


//Render Appropriate
HPDF_Page_DrawImage  (*BlitParams->page,
                        WorkingGraphic,
                        BlitParams->OutputRect->X,
                        BlitParams->OutputRect->Y,
                        BlitParams->OutputRect->Width,
                        BlitParams->OutputRect->Height);

Does anyone know how to drive libharu so it creates sensible sized pdf's when you use .png images?

Solution

Right I don't know how to remove a question but maybe this info will be useful to others anyway.

I may have had the same issue as this fellow here where I have duplicated this answer.

What I needed to do was enable compression of the .pdf, which I had not done.

Documentation link

C Code:

HPDF_SetCompressionMode (pdf, HPDF_COMP_ALL);

It's because I didn't do enough research to know that .pdf format does not natively support .png, or if it has been updated to do so, libharu still doesn't. So, this option tells libharu to use zlib to zip compress everything it can, including your images.

The implementation is not perfect (you will still see a size difference if you zip your output .pdf) but it is acceptable for my use case.