setScale

sets a new value for zooming scale and renders the current page with the new scale

void setScale(string|number scale,boolean update);

scale	string\|number	a new scale value
update	boolean	if true, updates the scale value on the related toolbar

webix.ui({
    view:"pdfviewer", 
    id:"pdf", 
    toolbar:"toolbar", 
    url:"binary->files/WebixDocs.pdf"
});
 
$$("pdf").setScale(0.5,true);

Details

For legal values check the scale property.