File size not reducing after compressing

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In my laptop, whenever i try to compress any file (doc, docx, pdf, jpg, etc etc) using 7zip or any other software, the size of compressed file remains the same as original file. I set the correct setting but still it's same. Some say that the file can't be compressed anymore and try to compress other file but i tested all my files in my laptop and none of those files are getting reduced in size after compression.

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Mark Adler On

Not sure about .doc, but .docx, .pdf. and .jpg are all already compressed. Something that's already compressed looks like random bits to a compressor, and so can't be compressed. In fact, they will be expanded very slightly. The only way to compress those is to first decompress them, and then recompress them with something that's more effective. Assuming that you can find something that's more effective.