This mentioned original, small file size, i assume, is from the MS Word / Excel document attached. If such a document is changed in OpenOffice, the saved file is about the double size of the original; after saving it in MS x again, it is reduced to the original size again. When converting to PDF, it is the safest option to use images, not text, and a screenshot of a text file will always be much larger than the text file itself 😉
Of course, a utility converting or printing MS x files to PDF will use the text (you can copy text from the resulting PDF) if possible...
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