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What are you using to create the PDF? If Acrobat, you can go to File>Properties>Initial view to set the scale that it opens at.
Thank you for the reply. Let me make my doubt a bit more clear. I wanted to know how the Adobe viewer scales images, say I put a 100x100 image and while adding in the PDF I give display width and height as 300x300, the PDF viewer scales the image to 300x300. I want to know how the viewer is scaling the image? Just pixel replication or some filtering?
I found the answer myself from the PDF standard. Image interpolation is done if the /Interpolate field in the image XObject is set true. I am quoting from the PDF standard below.
"When the resolution of a source image is significantly lower than that of the output device, each source sample covers many device pixels. As a result, images can appear jaggy or blocky. These visual artifacts can be reduced by applying an image interpolation algorithm during rendering. Instead of painting all pixels covered by a source sample with the same color, image interpolation attempts to produce a smooth transition between adjacent sample values. Image interpolation is enabled by setting the Interpolate entry in the image dictionary to true. It is disabled by default because it may increase the time required to render the image."