PDF is a vector-based format entity and there are no “classic” dots (pixels) as one may find in JPG, PNG, TIFF pictures. In opposite, PDF format internally uses something similar to computer programs that can draw lines, circles, text, forms, etc. So PDF file is automatically executed to create a picture for various DPI (Dots Per Inch) targeting screen or printing device depending on specific monitors or printing devices accordingly. This flexibility allows PDF documents to be printed and displayed with the highest quality even for super large sizes. If you compare this to JPG, PNG pictures then pictures are just a set of pixels and you will see blurring effects when pictures are zoomed in or increased in size.
So if you need to convert PDF into a JPG, PNG or TIFF picture then you should use PDF Renderer SDK and just set size and resolution of output picture with a given resolution. And the SDK will render PDF documents into a picture with the desired picture resolution (300 to 600 DPI for high-quality pictures). So you should not worry about the resolution of the source PDF but just need to define output picture size with the same aspect ratio as the source PDF.
For the simplicity, when you are creating PDF document using tools like PDF SDK (PDF Generator SDK) then these tools are using coordinates calculated using 72 dots per inch (but this can be changed using .Resolution property in the SDK). But, again, this “default” DPI value in PDF Generator SDK tool is just for having a way to comfortably use coordinates to add lines, text, form elements, images (and other object types) and you may create thumbnail or print PDF using any other required resolution and output size.
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