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I would like to use pdf for ease of use and annotations when creating previews of my screen designs. However, no matter how I create my pdf, I can't seem to figure out what determines the 100% view of my pdf file. Naturally, I would like my pdf 100% view to be the same size on screen as my photoshop layouts - how do you achieve that?

From my tests I can see that the dpi of the photoshop/tiff/jpg file affects what acrobat picks as 100% when importing the image from file (or saving as pdf from photoshop, for that matter), but seeing how about 95dpi for the image file results in a 100% pdf view that roughly is the same as my actual pixel size, I wonder what's the logic here.

Short answer as I understand it based on the answers: 100% size is determined by the "preferences > page display" ppi value, which is user/system specific

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Maybe because monitors are 72dpi? – Lauren Ipsum Nov 28 '12 at 11:48
@LaurenIpsum: the original Mac was. – e100 Nov 28 '12 at 16:42
Can I just make sure that you are saving PDFs from Photoshop images and not from say InDesign layouts? – e100 Nov 28 '12 at 17:08
@e100 yeah, I thought that might no longer be the case, thus the "maybe" qualifier and a comment rather than an answer. – Lauren Ipsum Nov 28 '12 at 18:13
@e100 I have similar results for a) saving pdf from ps, b) saving tif/jpg from ps and creating a pdf from file in acrobat. – kontur Nov 29 '12 at 8:05
up vote 4 down vote accepted

Every screen has it's own resolution. Or rather pixel density.

Consider a 20" (diagonal) monitor:

A monitor set to 2560x1600px has a ppi of about 137

A monitor set to 1920x1080px has a ppi of about 102

A monitor set to 1440x900px has a ppi of about 89

I post "about" because actual physical size of the monitor is a factor as well. A 20" monitor with a 2560x1600 resolution will have a higher PPI than a 30" monitor at the same resolution. (This is the entire theory behind "retina" displays - huge resolutions on a small screen).

If you create a document at 300ppi and tell Acrobat to view it at 100%, Acrobat takes into account the monitor's pixels per inch and tries to match the monitor ppi to the document ppi. So a 300ppi document on a monitor with a pixel density of 100 would show 3 pixels for every 1 pixel of density. A 100ppi document on a 100 pixel dense monitor would be a 1:1 ratio. This is how Acrobat determines "100%".

If you want to accurately display sizes in Acrobat you need to alter Acrobat's preferences. If you go to Preferences > Page Display there is a field to input Custom Resolution. Input your monitor's PPI in that field and 100% will be much closer to actual size regardless of what a document PPI is. You'll also see an option in Page Display preferences to simply use the system settings. You can use that as well if it is reading a value. Not all systems will have a default value other than the standard 72/96ppi however.

For a close estimate at your monitor's pixel density you can use this calculator: Pixel Density Calculator

Note, the calculator will only get you close. It's not 100% accurate. For example it tells me my Monitor PPI is 100.63. However, using 100.63 in Acrobat causes all kinds of display errors. So I rounded to 101. If you want to be 100% accurate in the pixel density calculations you need to do the math yourself since every monitor manufacturer will have a slightly different variation for a monitor's size. The wiki article I linked to at the top of this answer explains the math behind pixel density.

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Essentially that means I can't enforce that a pixel image being converted to a pdf gets viewed with pixel accuracy at 100% across different devices, no? – kontur Nov 28 '12 at 12:39
No. Each devise has it's own pixel density. In the digital world 100% is NEVER the same. it's different on each and every screen which displays it. – Scott Nov 28 '12 at 13:00
The maths is trivial if you have a ruler and so can measure your screen though. It's just (vertical pixel count) / (vertical height measured with ruler). Or the horizontal equivalent instead - it'll give you the same value. The reason diagonal measure is brought into it that it's the measure that manufacturers quote. The Wikipedia article is rather overcomplicating it. – e100 Nov 28 '12 at 16:49
Also, it's worth underlining that other users' Acrobat display PPI value won't be at all accurate unless they have taken the trouble to set it properly - as Scott says, it'll probably default to 72 or 96. It might be worth talking your clients through adjusting it for their own monitor. – e100 Nov 28 '12 at 17:00
Agreed, just because you set your system correctly, it doesn't mean other systems will are set correctly. – Scott Nov 28 '12 at 18:45

If you're trying to match the size of the graphics from Photoshop to PDF on the same device, you shouldn't have any problem: 100% in Acrobat is the same as Print size in Photoshop. Both apps adjust your display size according to the dpi with these view settings.

In the end, Acrobat really isn't intended for web work. It's geared around faking your print output on screen. Within your P'shop doc, you can preview what Acrobat will try to do by looking at the Document size ...

Acrobat tries to fake the width and height shown here.

For web work, I usually check my designs on three different monitors with progressively degrading pixel density. If the project will be seen on mobile or tablet devices you have a really long list of possibilities on your hands ;)

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Ahh but Photoshop also has a pixel density setting.... Document PPI is not really relevant for screen display. 100 pixels x 100 pixels is exactly that on screen, regardless of whether the image is 300ppi or 72ppi. – Scott Nov 28 '12 at 18:35
Now whether 100% equals 100 pixels is all dependent upon the pixel density of the display. – Scott Nov 28 '12 at 19:02

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