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

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.

I needed this information and just found it here:

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 use the following command:

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

and it returns:

Size: 380x380