Welcome debbs. . . .with a PDF document there is limited adjustment that can be made for reading. The Kindle
has a PDF reader, but it's not particularly full featured. None of the usual font size/style adjustments will be available.
To get those features you need to convert it to Kindle format. This will also mean you'll lose any specialized formatting. There are a number of ways to do it.
1. Send it via Amazon to your kindle's 'send to' address with the word 'convert' as the subject and the file as an attachment. Send it to the @kindle.com address and it will go wirelessly but you may be charged if it's delivered via 3G. Send it to the @free.kindle.com address and it will be converted and returned to you for you to sideload it. Or will be delivered wirelessly next time you connect via WiFi. Information on the 'send to' address for your device is on the Manage Your Kindle page under Personal Document Settings. (Actually, if you've not gone all the way through the various MYK pages you should just to familiarize yourself.

)
2. Convert it yourself using a 3rd part software solution. There are a number of free ones available including Calibre and MobiPocket Creator. Google is your friend.

Once you've converted the file, you can sideload it or send it as suggested above except you won't need to say 'convert' because it will already be a compatible file type.
Note that if the book in question has DRM, a form of copy protection, neither of these methods will work. But if you were able to open the PDF file on the Kindle I'd guess that's not the case.