The pdf file I use is high quality, and if you were to zoom at a letter as much as Adobe Reader can handle, you still find it difficult to look at a blocky pixel at the edges of a letter. But when I convert to PNG, it will zoom and the letters will look very pixely, it's as if it only saved the details that can be observed at its normal display size. How can I fix this? imagemagick is a last choice, if there's a different program tell me
How can I fix this?
The formats are inherently different and work in different ways.
PDF supports what is known as "vector" graphics. PDF allows you to specify line and curve start and end points and you can effectively "draw" items perfectly. This is why you can zoom in infinitely without loosing any quality in the picture. Lines are redrawn at your current resolution and zoom and always look sharp. In vector graphics there is no real concept of resolution, only relative co-ordinates and scaling.
PNG on the other hand is a "raster" format. It supports stating what colour each individual pixel is and that is it. The "resolution" of the file tells you exactly how many pixels there are in it. When you zoom in you start seeing individual pixels. To see the same level of detail as the PDF you would need an infinitely large file and it would be difficult to manage and work with.
You can also embed raster graphics data such as JPEG images into a PDF, but that is a story for another day.
If you want to retain the vector format of the PDF you can import it into a program such as Inkscape and export it as an SVG or EMF file which also support vector graphics.
During the conversion, you should be able to select a target resolution (if not, use a different tool / process).
Increasing it will result in better resolution, but larger file size;
reducing it gives poorer resolution, but smaller PNGs.
Note that once you made the conversion, the target has a defined resolution, and if you just zoom far enough, you will see pixels again. PDF offers an inherently different approach - it stores the letters, not the pixels, and the displaying program knows how the letters have to look in a certain font, so it can offer nearly limitless 'zooming' without pixelation.