If you want to change the icons for the buttons, you would need to upload new images and point the CSS to those images. The icons are located in the /img/ directory of the plugin and the CSS to call those images:
/* DOWNLOAD BUTTON IMAGE CSS */ div.pdfemb-toolbar button.pdfemb-download::before { content: url(../img/toolbarButton-download.png); } /* ZOOM OUT BUTTON IMAGE CSS */ div.pdfemb-toolbar button.pdfemb-zoomout::before { content: url(../img/toolbarButton-zoomOut.png); } /* ZOOM IN BUTTON IMAGE CSS */ div.pdfemb-toolbar button.pdfemb-zoomin::before { content: url(../img/toolbarButton-zoomIn.png); } /* PREV PAGE BUTTON IMAGE CSS */ div.pdfemb-toolbar button.pdfemb-prev::before { content: url(../img/toolbarButton-pageUp.png); } /* NEXT PAGE BUTTON IMAGE CSS */ div.pdfemb-toolbar button.pdfemb-next::before { content: url(../img/toolbarButton-pageDown.png); } /* FULLSCREEN BUTTON IMAGE CSS */ div.pdfemb-toolbar button.pdfemb-fs::before { content: url(../img/toolbarButton-presentationMode.png); }