Moving from General to Images, this is the panel in which you’re going to have the most significant impact on your file size. What this panel does is it determines what’s going to happen to Color, Grayscale or Monochrome images that are in your PDF file. It turns out that, things like photographs whether they’re color or grayscale, or even pixel-based images that don’t look like photographs, maybe logos and things like that, add the most to a file size. And by controlling that here when you produce a PDF file, you’ll place yourself on that continuum between small file and high fidelity.
Let’s take a look how I would set this up and it will help you understand what settings you should choose for the situation that you’re looking for. We’ll start with the small end of the spectrum. I want to create the smallest file possible. The first thing that I want to do to reduce file size is reduce the number of pixels in my color images. To do that, I’m going to choose Sampling, and I’m going to leave By Cubic Down Sampling selected. What that’s going to do is it’s going to remove pixels from the image and then sort of re-jigger the image if you will to look like it did when it when into the PDF file but it will have far, far fewer pixels. How many pixels do I want to get down to? Frankly, if I want the smallest file possible. I’m going to go to 72. 72 pixels per inch is enough so that it will look pretty good on screen but it will be nice and small.
Now for images above, when I hit the tab key, you’ll see that it automatically fills in with a hundred and eight. That’s just a default value that Acrobat put in there, but frankly, I’m trying to make the smallest file possible so I’m going to enter 72 pixels per inch. And what that means is that any image in this PDF file that goes in as anything more than 72 pixels per inch of resolution is going to come out as 72 pixels per inch of resolution. Even if it only went in with 79 pixels per inch of resolution, it’s still going to squeeze some size out of the file. That’s the first way to make your files smaller. Understand though, that if you were to take this file and go to even an Inkjet printer, you’re going to see what looks like jaggedness especially along curving lines and things like that in the photograph. This is meant for looking at the PDF file on screen only. It’s not going to be meant for printing.
Now there’s one more impact that I can have on my pixel-based images and that is through compression. Compression is the process of looking at the pixels in the image and doing something to them to try and make the file even smaller still. And in the world of compression, there are two options and they’re represented here by JPEG and ZIP. The first option, JPEG, is what’s known as lossy compression. Lossy compression is the process of actually changing the pixels in the image so that it can create a Mathematical model of that image and then represent it using numbers. If the JPEG compression scheme can do that, it can give you a much smaller file than just having pixel after pixel with color values attached to them that it has to describe each and every one of very painstakingly. Lossy compression is excellent for color photographs but remember, it will change the look of the photograph so the more compression I add, the smaller the file, the less quality I’m going to see in the image.
Now the other option is ZIP, and ZIP is known as non-lossy compression which might seem to be a better option. It’s not going to affect the image. The reason that JPEG is more typically used for color images, is because the way ZIP does its work, it doesn’t actually do much to a color image compression lines. In fact, in some cases it can actually make the file larger because of the overhead involved with using the ZIP compression for color photographs.
Now if your document has a lot of flat color, a lot of pixel-based logos that really don’t vary in color, you might try ZIP compression because it mighty actually provide you a better result but for the most part, we’re talking about images, photographs, things like that so JPEG is the compression scheme you’re going to want to choose.
How much JPEG is the third option that you’re going to want to consider and that is found under Image Quality. The higher the image quality, the less compression you’ll see. So a good happy medium, no pawn intended, is the medium quality setting. Again, I’m trying to make a file here that is as small as possible and because I’m sharing with somebody who knows that, they’re not going to be too upset by medium quality JPEG compression. It will show up. If you look at the file, you’ll see that kind of speckling that you see around edges, and if you want to see a lot of examples of JPEG compression, go to the internet some time and look at photographs. Those typically have been very, very highly compressed so that it doesn’t take too long to deliver them to your browser from the web server. And in doing so, it creates this compression artifacting I call it, which sort of looks like speckling around edges.
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