Musings on Photography

PDF Size

Posted in PDF by Paul Butzi on February 21, 2010


Since switching my static website over to having PDF portfolios instead of web galleries, one of my concerns has been that some of the portfolios are quite large (e.g. 28 MB). That’s bad because it makes them slow and expensive to download, and of course if they’re larger a lot of server bandwidth is consumed as well. Even worse, when the portfolios are large, you get a lot of folks who download part of the portfolio, get tired of waiting, and abandon the download – that bandwidth is completely wasted and the portfolio never gets viewed.

So I’ve been experimenting, seeking ways to get the size of the portfolios down without cutting the number of images in each portfolio.

One way to get the size down is to reduce the pixel pitch of the images. If you generate a PDF with images at, say, 150 PPI, the file will be larger than if you have the images at 100PPI. The downside to reducing the pixel pitch is that the images start looking worse quite quickly, and you limit the size of display at 1:1. I can see the loss of detail pretty rapidly when I start reducing the pixel pitch.

Interestingly, based on some experiments that Martin Doonan did, I started experimenting with increasing the compression. In InDesign, compression is controlled with the ‘image quality’ choice. Switching from ‘maximum’ to ‘high’ reduced the size of a 28 MB pdf down to about 19MB, quite an improvement, and I couldn’t see a difference comparing the two versions on screen. Going from ‘high’ to ‘medium’ got the file size down to 10 MB, and I *still* couldn’t see any degradation. Oh, I’m sure if I went through every image in the portfolio, and spent minutes comparing the images at various sizes on screen, eventually I’d find the differences.

But that’s not really how PDF portfolios get used, I think.

Bottom line: if you want to get the size of your PDF’s down, and you want to keep the PDF’s viewable on large screens, don’t reduce the pixel pitch. Get in there and start increasing the compression. You can cut the size of the PDF down to 30% of the size and have minimal effect on quality of display.

As a side note, I’d give a lot for a tool that would let me switch rapidly back and forth between two PDF files, to make it easier to compare.

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5 Responses

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  1. doonster said, on February 21, 2010 at 7:46 pm

    As you’ve recreated my experiments, it draws up two observations:

    1. How valuable is it to enable zooming of images? That’s, of course, a personal decision. I prefer not to zoom and prefer to present PDFs notto be zoomed. So lower pixel pitch works for me.

    2. It looks like compression factors for the images and the result on image quality may well be software dependent. I certainly couldn’t make those same jumps in subjective comression factor (the H,M,L scale) compression using Scribus. Why can’t programmers give us the actual jpeg compression factor (as a number) so we know what we’re getting into?

  2. Paul Bradforth said, on February 22, 2010 at 1:54 am

    Paul, I saw this on the Daring Fireball blog yesterday; I don’t know if it’d be helfpful:

  3. Rosie Perera said, on February 22, 2010 at 7:29 am

    > As a side note, I’d give a lot for a tool that would let me switch rapidly back and forth between two PDF files, to make it easier to compare.

    Well, if you hadn’t switched to Mac I’d have said “what about opening both PDFs in Adobe Acrobat Reader and just Alt-Tabbing back and forth quickly between the two of them?” Is there nothing equivalent on the Mac?

  4. Dennis Allshouse said, on February 22, 2010 at 3:28 pm

    How big are the images to start with in terms of pixels. BYW (Rosie) Abode Reader runs on the Mac.

  5. Paul Butzi said, on February 22, 2010 at 3:48 pm

    The pages for the PDF document are formatted to be 10″x12.5″.

    The images, as embedded in the InDesign document are 8″x12″ at 300ppi, or 2400×3600 pixels.

    So when InDesign resamples it’s reducing the image by a factor of two.

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