Midline - The imaginary line that rests on top of the body of the lowercase letters, disregarding ascenders. The distance between the baseline and the midline is the x-height.

PDF - (Portable Document Format) - a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. 

Serif - A short line or finishing stroke that crosses or projects from stems or strokes in a character. Serifs have many shapes, including hairline, bracketed, wedge, and slab. Fonts without serifs are called “sans serifs.