In addition to text files and e-books of various formats, Amazon's Kindle reading device can display PDF files. However, in unconverted form, PDFs are unaffected by the Kindle's built-in font settings, which let you raise or lower the type size quickly. Changing the Kindle to Landscape mode may make it easier to read some PDF files, but if you want to use the font settings, you'll have to send the PDF to be converted by Amazon's automatic service.
Try changing the Kindle's view mode to "Landscape." This often makes PDFs appear larger and clearer. Open the PDF on the Kindle and press the "Aa" button next to the space bar. Use the directional pad to select one of the landscape modes from "Screen Rotation," and press the central button.
Convert the PDF to the Kindle's MOBI format if Landscape mode is still unsatisfactory. The easiest way to do this is to send an email to "firstname.lastname@example.org," where "username" is your Kindle username. You don't need to put any text in the body of this email -- just attach the PDF file and make sure the subject line is "convert."
Connect your Kindle to a wireless network -- from the main menu, press "Menu," select "Settings," then select "Wi-Fi Settings" to see a list of available networks -- and the converted PDF will automatically be transferred to the Kindle. If no wireless service is available, you can send the PDF to "email@example.com" rather than "firstname.lastname@example.org," in which case it will be downloaded over the 3G phone network for a fee of 10 cents per file.
Open the newly converted file on your Kindle and use the "Aa" button to change the font size, just as you would with any other Kindle document.