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I have these 2 JPEG's and I join them into a single pdf with the following command:

convert -compress JPEG -quality 90 -page A4 1.jpg 2.jpg doc.pdf

However the resulting document's size is only 50x71 mm, you can see it on the following image. This happens with every -page format. It works well if I don't specify a -page.

Full size capture

(No, this capture is not scaled down to fit in the question).

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up vote 1 down vote accepted

I don't know why this happens, but it turns out that I have to also specify a pixel density (DPI) using the -density command:

convert -compress JPEG -quality 90 -page A4 -density 72 1.jpg 2.jpg doc.pdf

I had to choose 72 because that was the only density that gave me the proper A4 size (210x297). Manually specifying 300 would give me the same small document: 300 is the DPI with which I scanned my images.

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