Gill Sans Light is a commercial font. As I responded below to ms_baker's response to you, this is not a freely downloadable font or even one that you can legally (or even easily technically) share with a MacOS user.
Although Microsoft bundles some faces of Gill Sans with Microsoft Office (not Windows itself), the “Light” face is not so-bundled.
Your choices are as follows:
(1) License the Gill Sans Light from a font foundry via such websites as font.com (<http://www.fonts.com/font/monotype/gill-sans/light>).
(2) Find another font that resembles Gill Sans Light and license that. You may even find some freeware / shareware font that suits your needs.
In either case, be aware that unless you have the exact same font, there may be slight differences in the font's actual name (for example, Gill Sans Std Light as an OpenType version of the font) as well as in either the font's exact design, the available glyph complement, and/or the metrics. You will need to carefully check your layout for any such quirks after installing the font and making any changes (hopefully the documents use paragraph and character styles for font specifications) to accommodate differences in the font name, assuming your editing is being done in a layout program or a word processor. However, there is one big further complication. You posted your query on a forum associated with editing PDF files and I guess we assume you are trying to edit this text in Acrobat!! Generally speaking, editing text in a PDF file is dicey at best other than for very simple “touch-up” operations and you may find it is much easier to go back and edit the original document and then regenerate the PDF!
I know this is a royal pain in the tuchas, but this is the reason why we generally advise against using so-called “system fonts” for commercial work and do recommend standardizing on cross-platform OpenType fonts that can be readily licensed through reputable font foundries.
Good luck!
- Dov
PS: This is one of the reasons why some of us joke about “Font” being a four letter word beginning with an ‘F’ …