If you have any problem reading the small letters then maximize the screen by draw the titlebar to the top of the screen or just doubleclick the titlebar. After that zoom the screen to a maximum your monitor can handle using browser options. This works only on desktop and laptop computers.
If you are watching a tablet or smartphone zoom can be achieved by widening your fingers on the glass.
Unless you have defined another application to open PDF files, many of the files on this site open in the Acrobat Reader. Once a file is open on your screen try and hit the Ctlr+L keys to open the screen in the enlarged "reading" mode. It may take a while before large PDF files are downloaded (some of them are 80MB).