OOM kill for multi-page PDFs

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Because Cloud Run containers only have an in-memory filesystem, I thought it would make sense to just pipe the whole PDF through convert instead of saving anything to file. Turns out that was a bad idea as ImageMagick takes a lot of memory during conversion, probably because it keeps every page with full resolution in memory until it is finished.

This will easily OOM kill the container if the PDF has more than a few pages.