PDFs are converted at 96dpi as 1px equals to 1/96th of 1in
. So 7.5'' corresponds to 720 pixels and it means that a 720px wide image will be printed to PDF in 96dpi. If you want to print the image for example at 192dpi then simply use a 1440px wide image and retain the <img> container dimensions (i.e. 7.5'' wide).