Yes.  Note which PO form you use (may need to change a couple of them).  Go to modify forms.  Pull up the proper form.  It is in one of the sub-reports.  Make sure you return to the "Main" tab before saving, else you will lose the form.
 
Suggest you make a backup copy first.
 
We have taken it one step further and added a "cursive" signature at the bottom of the form, with Co. name & title etc.  Did one for each of the main people who buy, added their initials to the file name so they can just pull up their form when printing PO's.

When I am changing fonts, it is usually for specific items on the form, such as making our name larger & bolder.
You can do a global change to the whole form but you need to select all fields first:  Bring your form up.  Go to Edit at the top, select all.  Change your font.  Then save.  might want to save as something else until you know you have it the way you want it.