How to reduce the size of PDF

The question of “reducing the size of a PDF file” comes up often in these forums.

It is exceptionally important to understand why a PDF may larger than you expect. Some possibilities:

(1)     Your original document has a very large amount of content, especially vector and raster image content.

(2)     You did various operations on a PDF file in Acrobat and instead of doing a ​save as ​you did a ​save​. ​Save as​ rewrites the entire PDF file, doing various optimizations in the process. ​The simple ​save ​operation simply appends the changed pages to the end of the existing PDF file, by definition making the file larger and potentially much larger than the original PDF file.

(3)     You used various techniques in creating your PDF file, many of which are counterproductive, that by definition yield bloated (and slow to render) PDF files. An example of such a technique is “outlining text” (often recommended by some Luddite print service providers with late 20th century workflows who perpetuate myths about fonts causing problems when printing). Another is that of flattening transparency which often converts text into outlines or raster images when there is interaction between text and objects that are not completely opaque.

(4)     You created the PDF file using parameters that did not reasonably downsample raster images to a resolution appropriate for your printing and viewing needs.

(5)     You created your PDF file with parameters that forced embedding of full fonts as opposed to the subset of the glyphs needed to render your text.

(6)     You created your PDF file with a PDF version of less than 1.5. PDF 1.5 and later allows for object stream compression. Non-image data within the PDF file (including text and vector parameters and data) are internally ZIP-compressed within the PDF file, often dramatically cutting the size of PDF files that are primarily text and vector.

What can you do to proactively avoid bloated PDF files?

Regenerate your PDF from your original documents, making sure you set the downsampling and compression parameters appropriately. Optimal compression while maintaining high quality is ​Automatic (JPEG), ​Maximum Quality​. Minimum downsampling for reasonable print output is 300 dpi for images over 450 dpi (color and grayscale) and 1200 dpi for images over 1800 dpi for bi-level (monochrome) images. Remember for screen viewing, that the old recommendations of downsampling to 72 dpi or 96 dpi are totally irrelevant where screen resolutions are now often between 200 and 300 dpi (such as on iPhones, iPads, and corresponding Android devices. If this generates a PDF file that is “too large” for your needs, your choices are either (a) downsample to a lower dpi setting, yielding PDF files that will possibly print and/or display images poorly, (b) set the compression method to a lower quality setting, also yielding PDF files that will possibly print and/or display images poorly (typically with very nasty JPEG compression artifacts), (c) a combination of (a) and (b) which is the worst of both worlds, and (d) simplify your original document to use fewer and/or smaller raster images. You should also ascertain that PDF files are created with fonts embedded as subsets! (For obvious reasons, all fonts used in your original document should be subset-embedded in the PDF file to assure that text renders properly both on screen and for print!) And finally, you should ascertain that the PDF file version is at least PDF 1.5.

You can also use the ​optimization ​features of Acrobat Pro to achieve similar results after the fact although generally you get poorer image quality downsampling and recompressing imagery that is already in the PDF file than if you regenerate the PDF file from sources with the desired target downsampling and compression settings!

What you should ​not ​do includes (a) “refrying a PDF file” - printing to PostScript and distilling, (b) not embedding fonts, (c) flattening transparency, and (d) using downsampling and compression parameters that yield fuzzy-wuzzy, artifact-ridden images.

The bottom line is that you cannot legislate the size of a PDF file! The size of such a file is primarily dependent upon the amount and type of content in your original document used for PDF creation and secondarily, upon the parameters used for PDF generation. If your original content is totally text and vector graphics, generally speaking, there is much less wiggle room for cutting PDF file size (other perhaps than converting very complex vector objects to low resolution raster images with the inherent loss of quality for both display and print.