View Full Version : Changing DPI in a PDF

02-17-2010, 07:12 PM
I've spent the day working on a heavily-illustrated scanned book, OCRing it in Finereader then adding some finishing touches in Adobe Acrobat 4 (ancient version but it still works.) The problem was, Finereader kept crashing, the whole PDF was so large. So when a page would need tweaking, I would re-OCR that individual jpeg and insert that into the PDF in Acrobat. After hours of tweaking, I thought I had great results, but then I decided to try viewing it in two-page view so that I could see both halves of an illustration on two pages. And then found that pages that I had re-OCRed were displayed at a different size than the original OCRed pages. The actual images are the same size, but Acrobat is interpreting them as being different because it thinks that they are different DPIs.

So, is there a utility that I can use to make all pages in a PDF be read as a specific DPI?

02-18-2010, 07:27 AM

In later acrobat versions (at least) you can do that by recompressing the file, the procedure in acrobat 8 it would be:

1 - make a copy of your pdf file and use that copy;
2 - choose advanced and pdf optimizer and you will get a window with 3 options - color, grayscale or monochrome images;
3 - on the above - or more than one if you have pages with different options - set the maximum pixels/inch to something less than the resolution you know the less resolution pages have (ex: if you have some with 300 pixels/inch and some 200 pixels/inch, you should set the maximum to 190 pixels/inch) - this will guarantee that just the pages you want are recompressed;
4 - in the box of the center you put the resolution you want your paged to be recompressed to.

Another possibility is to do it in Finreader itself, you can do it from the original files or reloading the final copy of the pdf I advise you to do (in 1) above.
Note that I do not know what version of Finereader you have but in version 9:

1 - after opening your pdf file, choose save as pdf;
2 - in the windows that opens to save choose options;
3 - from the window that opens, choose picture settings and from the options available choose custom;
4 - from the window that opens choose one resolution that is less than the lower one you want to compress. If your smaller resolution is less the 96 dpi you will not be able t resolve your problem this way.

Best regards,

02-18-2010, 02:15 PM
Thanks for the tips, but I still haven't found a satisfactory solution. I redid the individual pages that were a problem in that one document, but it looks like all the PDFs I've been created have been being reported as being what looks to be 72dpi (the typical figure used for "screen resolution") when they were scanned at 300dpi. Which explains why I was having a problem viewing the final PDFs on my Sony Reader-- it would look great (but small) on the full-page view but the text would be massively oversized when I reflow to medium font, and too big to show up at all at large font.

I don't want to redo all the PDFs, I don't want to throw away resolution-- all I want is to modify the PDF file so that it knows that those "72dpi" images are really 300dpi, therefore from pages around 6 inches across and not pages around 25 inches across like the PDF thinks that it is. I don't see any reason why that is something that could not be easily fixed-- change a couple of bits in the settings of the PDF (or at worst tell it to change a couple of bits on every page.) But it doesn't seem like Acrobat (or anything else?) is willing to do that simple fix.

02-18-2010, 02:39 PM
Acrobat itself doesn't have a way to do this (that I know of); there are plugins that will. Quite A Box Of Tricks will adjust DPI, but it's not a cheap plugin.

FineReader should let you export with a choice of DPI options; you should be able to re-output the pages with different settings. (Or, hrm, the difference happens when they get loaded *into* FR; you need the images going into FR to be the right resolution.)

You should be able to save out/extract JPGs from a PDF with your choice of resolution. (You can in Acrobat 7; I don't remember if you can in 4.) Otherwise, save out the pages and then adjust the DPI with another program--maybe Photoshop (not free), GIMP ( (free, complex), or Irfanview ( (free, simple).