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What files do not reduce size after compression

I wrote a java compression program. I compressed some text file. The file size after compression has decreased. But when I tried to compress the PDF file. I do not see file size changes after compression.

So, I want to know which other files will not reduce its size after compression.

Thanks Sunil Kumar Sahoo

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16 answers

File compression works by removing redundancy. Therefore, files that contain little redundancy are compressed badly or do not work at all.

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Otherwise, the set of all input sequences up to a given length L can be mapped onto the (much) smaller set of all sequences of length less than L and do this without collisions (since compression should be lossless and reversible), which excludes the possibility of the pigeon principle .

So, there are endless files that do not reduce its size after compression and, in addition, the file does not require a file with high entropy :)

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