: Check image compression used in PDF document? Is it possible to find out what compression algorithms have been used for images in a PDF document?

@Goswami567

You can use pdfimages -list $file on Unix/Linux systems, on Windows maybe you can install poppler to get access to the tool (see en.wikipedia.org/wiki/Pdfimages). Note that this is a command line application, without graphical user interface. Its output will look like

page num type width height color comp bpc enc interp object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
1 0 image 1240 1753 gray 1 8 image no 8 0 150 150 301K 14%
2 1 image 1240 1753 gray 1 8 image no 22 0 150 150 281K 13%
3 2 image 1240 1753 gray 1 8 image no 36 0 150 150 336K 16%


Where the enc column tells you the encoding algorithm, the possible values are listed in the manpage for pdfimages.

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@Sims5801359

Sort of.

If you open a PDF in a text editor, you'll find a line like this for each image:

<</Subtype/Image/Length 7986/Filter/FlateDecode/BitsPerComponent 8/ColorSpace 34 0 R/Width 368/Height 110/Type/XObject>>stream
...


This image uses the FlateDecode filter, "a commonly used filter based on the zlib/deflate algorithm (a.k.a. gzip, but not zip)"

<</Subtype/Image/Length 892/Filter/DCTDecode/BitsPerComponent 8/ColorSpace 34 0 R/Width 44/Height 23/Type/XObject>>stream
...


This image uses the DCTDecode filter, "a lossy filter based on the JPEG standard"

Filter definitions are from the Portable Document Format Wikipedia page, which has a list of all the supported filters.

However, it can be difficult to work out which image is which in a text editor - the objects do not necessarily occur in reading order. Breaking the PDF into single pages might help here.

I think Enfocus Pitstop Pro, an Acrobat plugin, may well give you this info (along with dimensions, position, DPI etc) in its object inspector, but I no longer have it, and can't seem to find any mention of this specific attribute in their manual.

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