My e-book library consists of PDF files mainly. Fortunately, Kindle supports PDF files as well as other formats including DOC, DOCX, PNG, BMP and a few more. Thus, you can view PDF files in your Kindle and zoom in/out if you want.
This feature will be enough for you, if the PDF file has narrow text columns that fit to the Kindle screen properly and font size is sufficently large to be able to read easily, But this is not the case for all PDF files even in landscape mode. Yes, you can zoom in, but column doesn't fit to screen this time. So, you need to be able to increase font size of PDF keeping column size fixed for reading comfort. Just as with Kindle (AZW) file formats.
Solution: Converting PDF to Kindle (AZW - AZW3)
Method 1: Mail to Kindle Store with "convert" subject
Amazon gives you an email address like [name]@Kindle.com to allow you sending your documents to your Kindle from anywhere. When you send a file to this mail, it will be loaded to your related Kindle automatically whenever the Kindle connected to the Internet. If you send the PDF directly to this mail, it will be loaded as PDF also to your Kindle. But if you send the mail putting "convert" word to the subject, then it will be loaded in Kindle format and you will be able to play with layout and font.
Method 2: Using Calibre
If conversion result from the Method 1 didn't satisfy you, then try second method: The Great "E-Book Management" Software: Calibre (free, multi-platform).
Load your book and right click it, then select Convert individually. (you can convert multiple files in once using Bulk convert).
A window will pop up to adjust conversion settings. Select desired output format. This will be AZW in this case. Click OK. Wait conversion to complete. Copy your file to Kindle using USB cable.
If result is still not sufficient, retry after enabling Heuristic Processing.
Note: If you converted PDF file using Calibre and your Kindle doesn't show it, then check your firmware version and update if necessary.