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Optimizing PDF Files

Posted on September 12, 2008 by Bo

PDF files that are created for press and printing can have high resolution and large file size. This can make it difficult to view these files on the Web or send them via email. In some earlier versions of Acrobat, you could simply go to File > Reduce File Size and you were done. The Reduce File Size option was a quick and easy way of downsizing your PDFs, but with the introduction of Acrobat 8 we’re presented with a much more full-featured tool; the PDF Optimizer.

Keep in mind that performing this Optimization can result in reduction of resolution and compressing certain file components. This is not recommended for files that are bound for press.

1. Save As
Before we get to the Optimizer, let’s first open your existing PDF file and look at the file size (File > Properties, or Ctrl/Cmd D). In the Description tab, look in the Advanced section and take note of the current file size. Now, do a simple Save As (File > Save As…, or Ctrl/Cmd + Shift + S) for the file. Examining the file size again may show a significant decrease; the Save As option will often remove unneeded components from the file without degrading the output quality.

2. Audit Space Usage
Now we can open the PDF Optimizer by going to Advanced > PDF Optimizer… The first thing you’ll want to do is click on the “Audit space usage…” button in the top right corner of the menu. This will give you a breakdown of the contents in your PDF and how much space they’re taking up. Images will typically be the items that are taking up the most amount of space.

3. Images
Hit OK to get back to the PDF Optimizer menu and make sure that Images is the option selected on the left. Here you can define the resolution that you want all images converted to. Use the following settings:

  • Color Images: Bicubic Downsampling to 100ppi for images above 100ppi.
  • Grayscale Images: Bicubic Downsampling to 100ppi for images above 100ppi.
  • Grayscale Images: Bicubic Downsampling to 300ppi for images above 300ppi.

The image Compression that you use will determine the amount of tonal loss your file will experience. To retain the most image data, change the Compression setting to “ZIP” compression (this works great for black and white images, as well as grayscale). Using “JPEG” will give you the most substantial compression, further reducing the file size. Choosing Medium Quality can often be used for viewing on screen while High or Maximum will work better if you intend to print to a consumer printer.

In Acrobat 9, you can check the option to “Optimize images only if there is a reduction in size” box if you want to retain the existing quality of your images if the compression will not result any file size reduction.

4. Fonts
In any PDF, you want to make sure your font files are preserved. Click on the Fonts selection to the left of the Optimizer and check the “Do not unembed any font” box (Acrobat 9 only). For Acrobat 8, simply make sure that all fonts listed are under the “Embedded fonts” box.

5. Transparency
The Transparency option may be beneficial if you have a considerable amount of transparent objects within the file. Simply check the box beside the Transparency option and choose “High Resolution,” as this keeps the Optimizer from converting your text to outlines, but doesn’t override the settings in the Images dialog.

6. Discard Objects
Next we’ll visit the Discard Objects menu. Items such as JavaScript actions, form fields and components, and alternate images can add on unnecessary file size to your PDF. Simply select the objects from the list that you wish to discard. The two selections that I would not recommend discarding are “Convert smooth lines to curves” and “Discard bookmarks.” The former would result in unexpected alterations to any lines in the PDF, and bookmarks take up very little space but can be helpful when browsing the document.

7. Discard User Data
The Discard User Data option is very similar to Discard Objects. You can manually go in and check which data you won’t need. Be sure to reference “Audit space usage” to make sure this data is worth removing. If it’s taking up a very small amount of space, then it might be safer to leave it.

8. Clean Up
The final option, Clean Up, allows you to go through and select document contents which can be removed or compressed. The very last option, “Optimize the PDF for fast web view” is the one you need to make sure is checked. This sets up the document so that when it’s being viewed on the web, the file is loaded one page at a time instead of loading all at once. This means your readers can be viewing the first page while the rest of the document is still loading.

9. Document Compatibility
In the upper right-hand corner of the PDF Optimizer menu, the document compatibility should be set to “Acrobat 5.0 and later.”

10. Finish Up
Now that you’ve got a good setup for the PDF Optimizer, click on the “Save” button in the top left corner to save the settings for future use. Now click OK to apply the Optimizer settings and save the file to your computer. Open the newly created PDF and go to File > Properties to view the changes in file size.

Extra Credit: Convert CMYK Images to RGB
For files that will be used on the web, you want to make sure the color profile of the images is RGB. With the file open, go to Advanced > Print Production > Convert Colors. The dialog box for this conversion varies between versions, but the main idea is to convert any and all CMYK (U.S. Web Coated (SWOP) v2) images to sRGB IEC61966-2.1. Lastly, check the option to Embed the color profile for optimum compatibility.


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