• Optimization Benchmark
• Benchmark Setup
• Results

Optimization Benchmark

One of the ways to use the hexapdf command is to optimize a PDF file in terms of its file size. This involves reading and writing the PDF file and performing the optimization. Sometimes the word “optimization” is used when a PDF file is linearized for faster display on web sites. However, here it always means file size optimization.

There are various ways to optimize the file size of a PDF file and they can be divided into two groups: lossless and lossy operations. Since all used applications perform only lossless optimizations, we only look at those:

Removing unused and deleted objects

A PDF file can store multiple revisions of an object but only the last one is used. So all other versions can safely be deleted.

Using object and cross-reference streams

A PDF file can be thought of as a collection of random-access objects that are stored sequentially in an ASCII-based format. Object streams take those objects and store them compressed in a binary format. And cross-reference streams store the file offsets to the objects in a compressed manner, instead of the standard ASCII-based format.

Recompressing page content streams

The content of a PDF page is described in an ASCII-based format. Some PDF producers don’t optimize their output which can lead to bigger than necessary content streams or don’t store it in a compressed format.

There are some more techniques for reducing the file size like font subsetting/merging/deduplication or object and image deduplication. However, those are rather advanced and not implemented in most PDF libraries because it is hard to get them right.

Benchmark Setup

There are many applications that can perform some or all of the optimizations mentioned above. Since this benchmark is intended to be run on Linux we will use command line applications that are readily available on this platform.

Since the abilities of the applications vary, following is a table of keys used to describe the various operations:

The list of the benchmarked applications:

hexapdf

Homepage: http://hexapdf.gettalong.org
Version: Latest version
Abilities: Any combination of C, S and P

We want to benchmark hexapdf with increasing levels of compression, using the following invocations:

None of C, S, or P

hexapdf optimize INPUT --no-compact --object-streams=preserve --xref-streams=preserve --streams=preserve --no-optimize-fonts OUTPUT

C

hexapdf optimize INPUT --compact --object-streams=preserve --xref-streams=preserve --streams=preserve --no-optimize-fonts OUTPUT

CS (so this would be the standard mode of operation)

hexapdf optimize INPUT OUTPUT

CSP

hexapdf optimize INPUT --compress-pages OUTPUT

Note that a lot of time is spent deflating when recompressing pages. This is because HexaPDF uses the highest deflate compression level by default. By changing the configuration option ‘filter.flate.compression’ to something lower than 9, it is possible to trade compression speed with file size.

origami

Homepage: https://github.com/gdelugre/origami
Version: 2.1.0
Abilities: ?

Similar to HexaPDF Origami is a framework for manipulating PDF files. Since it is also written in Ruby, it makes for a good comparison.

The origami.rb script can be invoked like ruby origami.rb INPUT OUTPUT.

combine_pdf

Homepage: https://github.com/boazsegev/combine_pdf
Version: 1.0.23
Abilities: ?

CombinePDF is a tool for merging PDF files, written in Ruby.

The combine_pdf.rb script can be invoked like ruby combine_pdf.rb INPUT OUTPUT.

pdftk

Homepage: https://gitlab.com/marcvinyals/pdftk
Version: 3.3.2
Abilities: C

pdftk is probably one of the best known applications because, like hexapdf it allows for many different operations on PDFs. It is based on the Java iText library. Prior version have been compiled to native code using GCJ but GCJ was deprecated and this fork of pdftk now uses Java.

The application doesn’t have options for optimizing a PDF file but it can be assumed that it removes unused and deleted objects when invoked like pdftk INPUT output OUTPUT.

qpdf

Homepage: http://qpdf.sourceforge.net/
Version: 10.4.0
Abilities: C, CS

QPDF is a command line application for transforming PDF files written in C++.

The standard C mode of operation is invoked with qpdf INPUT OUTPUT whereas the CS mode would need an additional option --object-streams=generate.

smpdf

Homepage: http://www.coherentpdf.com/compression.html
Version: 1.4.1
Abilities: CSP

This is a commercial application but can be used for evaluation purposes. There is no way to configure the operations done but judging from its output it seems it does all of the lossless operations.

Invocation is done like this: smpdf INPUT -o OUTPUT.

The standard files used in the benchmark (not available in the HexaPDF distribution) vary in file size and internal structure:

Results

These benchmark results are from 2023-08-03.