From http://aur.archlinux.org/packages.php?ID=52333

--- mupdf-0.9/apps/x11_main.c
+++ mupdf-0.9/apps/x11_main.c
@@ -539,6 +539,21 @@
 	exit(1);
 }
 
+int get_dpi(void)
+{
+	Display *xdpy;
+	int xscr;
+	int x = 0;
+	
+	if ((xdpy = XOpenDisplay(NULL))) {
+		xscr = DefaultScreen(xdpy);
+		x = (int) (DisplayWidth(xdpy, xscr) * 25.4 /
+		DisplayWidthMM(xdpy, xscr) + 0.5);
+		XCloseDisplay(xdpy);
+	}
+	return x;
+}
+
 int main(int argc, char **argv)
 {
 	int c;
@@ -547,7 +562,7 @@
 	KeySym keysym;
 	int oldx = 0;
 	int oldy = 0;
-	int resolution = 72;
+	int resolution = get_dpi();
 	int pageno = 1;
 	int accelerate = 1;
 	int fd;