imagemagick reduce size of pdf

Question

I need to automatically reduce the size of some user uploaded pdfs so that they can be sent via email.

I have a little imagemagick oneliner that reduces the size for me:

convert -density 120 -quality 10 -compress jpeg original.pdf output.pdf

basically exports every page of the pdf in jpg, updates density and quality and repacks the pages in a new PDF.

this works perfectly, except that with this command sometimes the files end up bigger, and I need to rerun tweaking density and quality to get the lowest size where the text in the pdf documents is still readable.

I'm not sure how to automate it. I thought to use identify to get characteristics of the files (height width density... ) and do stuff like half the figures or sth similar. but I'm struggling to get this info about the files.

Any suggestions?

Thanks,

Answer 1

Addendum: Parameter -compress jpeg must be explicitly submitted or else you will end up with uncompressed image assets in your PDF:

$ cd ~/Pictures/Scans/

$ pdfimages -list Test.pdf 
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image    4961  7016  icc     3   8  jpeg   yes        5  0   600   600 5907K 5.8%

$ convert -density 150 -quality 60  Test.pdf Test-150-060.pdf 

$ pdfimages -list Test-150-060.pdf 
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image    1240  1754  rgb     3   8  image  no         8  0   150   150 6397K 100%
   1     1 smask    1240  1754  gray    1   8  image  no         8  0   150   150 33.5K 1.6%

$ convert -density 150 -quality 60 -compress jpeg  Test.pdf Test-150-060-jpeg.pdf 

$ pdfimages -list Test-150-060-jpeg.pdf 
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image    1240  1754  rgb     3   8  jpeg   no         8  0   150   150 42.5K 0.7%
   1     1 smask    1240  1754  gray    1   8  image  no         8  0   150   150 33.5K 1.6%