In general, you can adjust the font size from within a book.
While a page is open tap along the top edge of the screen. One of the icons will look like Aa -- Tap it. There you can adjust line space, margin size, font size and font style.
SOMETIMES a book has a fixed font style and you won't be able to change that, but you should still be able to change the size.
If you don't have a touch screen kindle, there's similar set of options via the menu. Again, accessible while actually in a book.
It is POSSIBLE the book you're reading has a specified style and
size and you can't change it at all. If you purchased the book from Amazon, I'd alert them, because they do not want that. If it's from elsewhere, I'm not sure what else you can do.
If the file you're looking at is actually a PDF, you can't adjust anything on the Kindle. If there's no copy protection, you can convert it to Kindle format and then put it on the device and it should work normally. But, note that you'll likely lose any specialized formatting -- charts or columns, etc.