Problems should use the Courier font. Why? Every Courier font character uses the same amount of space. A comma is the same width as the number 5. This means that all of the numbers line up perfectly for carrying during addition and bringing the zero down in division.
Developers of free math worksheets primarily use either of two methods for displaying and printing worksheets on the Internet. PDF pages require an additional piece of software, Adobe Acrobat, which should automatically open worksheets in your browser. PDF worksheets theoretically cannot be manipulated and display/print precisely as the developer designed. The second mode of delivery, HTML code, displays worksheets directly in your browser window. The downside of using HTML based worksheets is they are prone to printing problems. A worksheet meant for one page can easily bleed over to a second sheet.