You may or may not continue to run into problems with Gill Sans depending on the age of the font file the original file creator used on Mac. Prior to OS X 10.6 Lion, the font was called Gill Sans. Later it became GillSans (one word) when it was incorporated into the Adobe library.
You can look online for a copy of the font you can freely download and install. It may allow you to edit as you like. If it doesn't, ask the original document creator to send you a copy of their font file, which you can install and use on Windows as Gill Sans is a TTF that's system-agnostic.
The Gill Sans family was part of the Adobe Type Library long before MacOS X of any 10.x version! Adobe licensed it directly from Monotype.
Note that your advice to “ask the original document creator to send you a copy of their font file, which you can install and use on Windows as Gill Sans is a TTF that's system-agnostic” has some major problems:
(1) There is a Gill Sans family that ships with the current MacOS X 10.11.x that in fact does include the Gill Sans Light typeface. However, although it is in a TrueType format, it is actually packaged in a Macintosh-format .TTC (TrueType Collection) file that cannot be directly opened or used on a Windows system. Inside that collection are nine different faces of Gill Sans.
(2) Gill Sans is a commercial font, even though it is bundled with MacOS. That does not give anybody the right to simply send a copy of the font file to someone else who is specifically licensed to use that font.
In terms of looking “online for a copy of the font you can freely download and install,” since Gill Sans is a registered trademark and a commercial product, you are not going to find such “free” copies unless they are illegally posted on a website. There are plenty of similar designs out there, some of which may be freeware or shareware, but they are not Gill Sans itself.
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.
PS: This is one of the reasons why some of us joke about “Font” being a four letter word beginning with an ‘F’ …
Hi Dov -
Thank you! I know that Gill Sans was originally a Monotype font, but certainly wasn't aware of its ramifications and licensing issues. I feel enlightened.