because the data is different.
a pptx file is secretly just a zip file containing a bunch of xml (basically the same as html) files describing the content of the document. you can even rename it to .zip and extract it. Its designed to be easily edited
a pdf is basically an image file designed for looking like it was printed. its designed to look the same on every device and sre very difficult to modify.
they are completely different internal structure.
a pptx to pdf converter just prints the pptx file and saves the resulting image as a pdf.
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