From the Inkscape wiki:

http://wiki.inkscape.org/wiki/index.php/Current_PDF_Support

At this time, it looks as if Inkscape only outputs uncompressed PDF output. Once you export a PDF, look at its file size. If it is too large for your application, then you have different options:

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epstopdf

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Export the exact same file to EPS (encapsulated postscript) and then use epstopdf to convert the file to PDF. This will make a compressed PDF.

[...]

epstopdf comes for example with a MikTeX installation; I found mine in MikTeX installation folder \miktex\bin\x64.

Then use in a command line; see also https://superuser.com/questions/599666/how-to-convert-eps-to-pdf-in-windows It may take some time; like 20 seconds or so.

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