How to reduce PDF file size without sacrificing quality

Online PDF Proofing Prepress 1 min read , May 28, 2021

by John Dean, Marketing Content Specialist, Enfocus

Print production teams are often faced with images with resolutions that are too low to reproduce with acceptable quality. However, there are also instances where PDF file sizes are very large due to extra data and images that are of a higher resolution than necessary.

Photographer: Kristina Tripkovic | Source: Unsplash

In training, we were taught that an image had to be 300 ppi at 100% to be printable. When we got out into the real world, the rule of thumb for image resolution used to be, make the ppi two times the dpi. This has changed over the years, especially when considering making plates versus digital print. This article will avoid discussing ppi vs dpi and let that argument lie. And it will presume that each print shop knows what image resolution it requires for plate making or digital output.

This page contains an image that is oversized. The effective resolution is 400 ppi, which is overkill for this A5 card. The PDF file size is 2.78 MB. This isn’t a big deal. But if a client supplied a 500-page PDF file that was already data merged for mailing, it becomes a big deal quickly. Especially when you consider that jobs can also be bogged down with complex vectors, as well.

Also shown above is an Enfocus PitStop Action List used to optimize PDF files. Paying specific attention to the images section, the resolution down-sampling can be edited to meet custom specifications. For example, this A5 card is being plated at 133 lines per inch, so continuous-tone images are going to be optimized to 266 ppi.

After running the Optimize PDF Action List on the file, the size is reduced from 2.78 MB to 1.68 MB. An in-depth look at what this Action List is doing reveals more than image resampling. The order in which optimization occurs is this:

  • Remove objects completely outside of the media box
  • Discard Objects (metadata, form fields, JavaScript)
  • Discard User Data (attachments, empty layers, dictionaries)
  • Clean-Up (Document structure, invisible objects, workflow data)
  • Images (Monochrome, Grayscale, Color)

All of these individual Actions are available in PitStop Pro and can be built into one Action List for optimizing PDF file size and reducing complexity. PitStop Pro recently added the ability to rasterize complex vectors, which will also contribute to improving output performance. Adding that process to the same Action List combines all the PDF optimization into one click or can be automated with PitStop Server.

Optimize PDF files as early in a workflow as possible for better performance and efficiency of downstream processes. Clean PDF files will contribute to simplified online proofing. With the PDF Review Module integrated into a Switch flow, you can position the optimization prior to automated online PDF proofing. Since the PDF Review Module displays PDF files as they would appear on press, it makes sense to perform preflight and fixes before proofing.

Automated PDF optimization keeps files clean and consistent throughout a workflow leading to repeatable, expectable results at output.

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