Size of the File Increasing While Win-zipping pdf. doc

I'll bet that the PDF files that you are not capable of compressing are scanned images of a document. The white space of a scanned document will not be one fixed value that can be compressed. (NULL for next 3 mm) There's also no repeating pattern that can be duplicated for compression. So the decompression reference table that must be buried into any ZIP file has little to no useful compression sequences but still increases the total size of the file.

Most JPG files have already been compressed so that visually the final image is still usable. JPG compression is a lossy compression. If you ZIP a JPG image this size growth effect is more prominent because a ZIP and a JPG table is buried in the document.

You appear to be forgetting that many file types are basically "already compressed".....

The rule is NEVER EVER try to compress a file twice. So if the file is already compressed, don't do it again. It will just get bigger. Some files are not called "compressed", but they are already as tight as they can be in normal use.....

The list of file types that are already as "tight" as they can be is very long, I am not going to attempt to list them all here. I probably don't know them all anyway, so it would be futile exercise....

The reason that a file gets bigger when already compressed is simply that the compressing software a) cannot compress further what is already compressed and b) it needs to add on its own "overhead"......

What I have talked about a above is "data compression", a data file cannopt lose any data, so the compressing software is under some strong constraints....

Some picture (compression) types will reduce the size of the file by losing some small bits, but the detail also suffers at the same time. You cannot reverse the process to get the exact level of detail back.....

.PDF files for example, are already heavily compressed, most cannot be made any smaller.....

I hope this helps....

I have not gone deeply into the mathmatics of data compression as it is not really needed to understand the problems.....also I am a bit rusty after 6 years of retirement!