How/Why to flatten a PDF before printing

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).

2. 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.