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Before I print a PDF file, a dialog box pops up that says "Flattening...", and it goes through each page repeating that step. I recently printed a large file (600+ pages) with a similar high-quality image on every page, and it took the dialog box forever (around 45 minutes) to go through each page. The printer was printing each page as it came through the network after flattening, so every few seconds another page would come out.

If I create a word document and put tons of text on each page without images, Adobe will still show a flattening dialog box, but just for a few seconds.

My questions are:

  1. Would the result (result meaning: the data being sent to the printer) of a printer-flattened document with text and a document with an image (let's say a high-quality image of the text) be similar in size? I would assume yes, because the printer prints an image of the text, with the same 300 dpi or so.

  2. If my assumption is correct, how do I go about flattening my large file down to what the printer would have done? I have tried using the flattener preview on the print production tool in Adobe Acrobat Pro, without any luck (it crashed my computer (32gb RAM/i7 10gen CPU) when trying to flatten 600 pages), and would crash Acrobat when even trying 1 page. Not sure why. I think that's the correct way, but I'm not sure.

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  • Feel free to ask me to clarify something if I wasn't clear
    – Dave
    Jul 14, 2021 at 21:18

2 Answers 2

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So a PDF is a container format that holds arbitrary chunks of binary data (even executable files); it also contains instructions for how to composite the arbitrary chunks of data. Flattening is not merely rendering to the proper resolution for the printer, but also includes things like transparency flattening and overprint (etc.).

PDFs do not have an inherent resolution and can have images of random/arbitrary PPI mixed with resolution independent vector data (fonts especially). It is possible to store a 300x300px image in a PDF set to .5x.5inches which is an effective resolution of 600. Alongside a 600x600 image set to 12 inches which is 50ppi effective.

  1. This stage is usually referred to as RIP. But it is unclear whether what is being sent to the print spooler is a full-frame raster image or if it is an intermediate scripting (like PCL). So this step is probably flattening, handling transparency etc, and then emitting something that may be much larger than 8.5 x 11 x 300 x 4 colors in bytes. It will not be "300dpi," but rather whatever the output resolution is. Consumer laser printers might emit something around 600dpi by default (note that while in the abstract, we often use ppi and dpi interchangeably; dpi is not ppi). The industry rule of thumb of 300ppi is tied to 150lpi halftone screens where we provide 2x the samples for accurate reproduction of a signal (Nyquist–Shannon sampling theorem).
  1. if you have the source file and software used to make the PDF, one can simply use a PDF preset that flattens the transparency and downsamples anything above x resolution (this will reduce effective ppi to something more sane if you have huge images (measured by pixels) with small target print size. I think exporting to PDF-1.4-version flattens everything. This also potentially reduces the PDF file size. If you do not have access to the source, then you can try "printing to PDF" from the PDF, using the same preset/options mentioned above. Older Acrobat had a print driver, for newer version one may need to use an "export" feature and export to PDF using the preset. One might also find open source Print-to-PDF print devices or PDF conversion tools (ghostscript comes to mind but may be overkill). You will have a second copy "pre-baked" that may print faster.
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  • You can experiment by printing a range of 10 pages or extracting a set of 10 to a new PDF and then playing with the smaller set of pages until you find the right process.
    – Yorik
    Jul 14, 2021 at 22:06
  • Interesting, so if I (not having access to the original file) re-export the pdf to PDF-1.4, It would not take long per page (no matter what type of content is on the page) to render for printer?
    – Dave
    Jul 15, 2021 at 4:33
  • In theory it will take less time because you are doing in "up front" once, rather than on-demand every time. So even if the initial flattening process is similar vis a vis time, you only pay that price once. You will still have the original if you are going to send it to a high-end printing press etc, so they can RIP it to their specs. Again, try it on 10 or 20 pages, time it, then extrapolate to 600 to see if it works.
    – Yorik
    Jul 15, 2021 at 14:36
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It all depends on whether you are printing one-off files or whether you are in a production printing environment. If you have a PDF file with literally thousands of documents in it, simple flattening won't really help to reduce the resources overhead created by fonts and artwork within the file, you might want to look at a professional tool to reduce these resources and ultimately the file size so you can print at full speed. Just running the file through the RIP, or save as reduced file size won't do much for a large print job.

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