When I open a letter size PDF and then zoom to 100%, the page physically displayed on my monitor is smaller than a real letter size sheet of paper.

How can I make "100%" on the computer screen correspond with "100%" in real life?

Details

This message suggests that I should be investigating the system-wide DPI settings for my monitor. xdpyinfo reports:

dimensions:    1024x768 pixels (271x203 millimeters)
resolution:    96x96 dots per inch

My monitor has a native display resolution of 1024x768 pixels and a diagonal display size of 12.07 inches. PX CALC returns the following information:

DPI: 106.05
Dot Pitch: 0.2395mm
Size: 9.66" × 7.24" (24.53cm × 18.39cm)

What I've tried so far

Running xrandr --dpi 106.05 successfully caused my PDF to appear actual size at 100%, but this effect was lost after rebooting.

To make the setting persistent I tried creating the following /etc/X11/xorg.conf:

Section "Monitor"
    Identifier   "ThinkPad X60 LCD"
    DisplaySize  245 183
EndSection

Section "Screen"
    Identifier "Screen0"
    Monitor    "ThinkPad X60 LCD"
EndSection

After re-logging in, /var/log/Xorg.0.log contained

[  1167.824] (**) intel(0): Display dimensions: (245, 183) mm
[  1167.824] (**) intel(0): DPI set to (106, 106)

but xdpyinfo still reported

dimensions:    1024x768 pixels (271x203 millimeters)
resolution:    96x96 dots per inch

and "100%" still appeared too small.

Link to XRANDR wiki

Link to making XRANDR changes persistant