I have some PDF files that I want to split apart into TIFF files using convert (in order to OCR via tesseract). This so far is working great - except that in order to automate the whole process, I need to set the DPI of the convert output. Right now, I am using a command like this:

convert -density 300 myFile.pdf -depth 8 -background white output-%04d.tiff

... which outputs the PDF files at 300 DPI. However, some PDF files have lower DPI (e.g. 150 DPI) which means that I don't want to output them at 300 DPI via convert - this creates excessively large TIFF files without any additional information.

I know that there are ways to check the DPI of images in a PDF file by opening Adobe Acrobat and messing around in the "preflight" tools. However, is there a way to determine via the command line the DPI of a particular PDF file?

3 Answers 3


Main answer

Since I am interested in the same kind of job (though not necessarily to OCR the PDF files, but to convert them to DjVu and then OCR them), I found this question and the responses lacking (since I needed to guess the DPI of the images with the number of pixels and then use the size as output by pdfinfo or other tricks---not to mention that the images inside a PDF may have different densities etc.).

After a lot of research more, I found that you can use pdfimages (from package poppler-utils) like the following:

$ pdfimages -list deptest.pdf
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
   1     0 image     100   100  gray    1   1  image  no         9  0    53    53  169B  14%
   2     1 image     100   100  gray    1   1  ccitt  no   [inline]      53    53  698B  56%

Notice the x-ppi and y-ppi at the listing above. It also lists the format in which the images are stored in the PDF, which is cool (sometimes, it is JBIG2, sometimes JPEG2000 etc.)

Note: The file deptest.pdf used above is available from pdfsizeopt's repository.

The real action

After that, you can simply extract the images with pdfimages itself or use pdftoppm (also from poppler-utils) to render entire pages in many formats that you may like (e.g., tiff, for scanning with tesseract).

You can use something like the following (assuming you have created a directory named imgs where you will put your images):

pdfimages -png Faraway-PRA.pdf imgs/prefix

The files will be created inside the directory imgs with names starting with prefix, as in:

$ ls 
prefix-000.png  prefix-047.png  prefix-094.png  prefix-141.png
prefix-001.png  prefix-048.png  prefix-095.png  prefix-142.png
prefix-002.png  prefix-049.png  prefix-096.png  prefix-143.png
prefix-003.png  prefix-050.png  prefix-097.png  prefix-144.png

You can, then, perform any surgery that you see fit with tools like scantailor or whatever you like.

More direct answer

If you just want to OCR a PDF file, you can use a program that is well-maintained and already packaged, namely ocrmypdf.

  • Note that x-ppi (x resolution in DPI) and y-ppi (y resolution in DPI) are NOT shown on the older versions of pdfimages which come with Ubuntu 14.04, for instance. What is available on Ubuntu 18.04, however, does include these values. pdfimages -v on my Ubuntu 18.04 machine shows I have version 0.62.0, which does have these features. Commented Nov 10, 2019 at 23:49
  • @GabrielStaples, thanks for pointing that out. I thought that Ubuntu 14.04 was already EOL'ed, but it "only" had its Standard Support ended July of 2019 according to wiki.ubuntu.com/Releases
    – rbrito
    Commented Nov 12, 2019 at 20:41

This technique also uses ImageMagick:

identify -format "%w x %h %x x %y" DAT_1.tif

The output is the size of the image and the dpi:

2480 x 3507 300 x 300
  • I would add a new line to the end of format, in case you want to do *.pdf to process all pdfs in directory. "%w x %h %x x %y\n" Commented Apr 6, 2018 at 23:43
  • This does not do what the main question asks. This returns the whole document size and resolution
    – Prescol
    Commented Jan 9, 2022 at 9:04
  • The site seems to be broken. the wayback machine only finds a domain parking from 6 May 2021
    – Cadoiz
    Commented Jan 25, 2023 at 9:57

I use the following command:

convert MyPDF.pdf -print "Size: %wx%h\n" /dev/null

and it returns:

Size: 380x380
  • 1
    Thanks - this gets the size of the pdf images (in your case, 380x380 as it is a square). The DPI is different. On my file that I just ran this command on, I get Size: 595x842 although the DPI (checking in Acrobat) is around 130
    – Jason
    Commented Apr 23, 2016 at 14:11

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