Merging PDF Files on Linux: pdftk vs poppler

PDF files have become the standard for sharing documents across devices and platforms. Whether you need to combine multiple reports into one file or append new pages to an existing document, having the ability to merge PDFs is essential.

On Linux systems, there are two popular command line utilities that allow merging PDFs – pdftk and poppler. In this in-depth guide, we‘ll compare these two tools to help you decide which is best for your use case.

PDF Merging Use Cases

First, let‘s look at some common scenarios where PDF merging is necessary:

• Combining multiple documents or reports into one file – Rather than sending various PDFs attached in an email, you may want to consolidate related documents into one PDF file for easier sharing and reading.

• Appending new pages to an existing PDF – When you have an updated version of a document, merging allows appending it as new pages in the original PDF.

• Assembling scans or images into a single file – If you have various scanned pages or images (JPGs, PNGs etc.), you can merge them into one PDF document.

• Building presentations and ebooks – PDF merging provides an easy way to create slide deck PDFs, ebooks, whitepapers and other documents from multiple file sources.

Introducing pdftk and poppler

Now let‘s look at our two PDF manipulation tools:

pdftk (PDF Toolkit) is a popular, dedicated command line utility for working with PDFs. It‘s not installed by default on most systems.

poppler is a robust open source PDF rendering library. The poppler-utils package provides command line PDF tools powered by this library, including pdfunite for merging PDFs. Poppler-utils is included in many Linux distro repositories.

With this background covered, let‘s dive into using pdftk and poppler for merging PDFs!

Merging PDFs with pdftk

pdftk is a flexible, lightweight tool specialized for common PDF tasks. Let‘s go through installing and using it step-by-step:

Installing pdftk

On Debian/Ubuntu systems, install pdftk using:

sudo apt install pdftk

On RHEL/CentOS systems, enable EPEL repository then install with:

sudo yum install pdftk

Or compile from source code.

pdftk PDF Merge Syntax

The basic command structure for merging PDFs with pdftk is:

pdftk input1.pdf input2.pdf ... cat output output.pdf

Where:

• input1.pdf, input2.pdf – Input PDFs to merge
• cat – Directive to concatenate/merge input PDFs
• output.pdf – Filename for merged output PDF

You can merge multiple input PDFs by listing each file separated by a space before the cat directive.

Now let‘s see this in action with an example.

Merging PDF Example with pdftk

For this example, I have two PDFs – report-pt1.pdf and report-pt2.pdf. The goal is to combine them into a single PDF called full-report.pdf.

The pdftk command would be:

pdftk report-pt1.pdf report-pt2.pdf cat output full-report.pdf

And that‘s it! The two PDFs have now been concatenated into the output file.

With just one line, pdftk provides a simple syntax for merging any number of PDFs.

Next let‘s look at the poppler approach.

Merging PDFs with poppler pdfunite

Poppler pdfunite offers a similar PDF merging capability, powered by the underlying poppler library.

Installing poppler-utils

On most Linux distros, poppler-utils is in the standard repositories. To install on Debian/Ubuntu:

sudo apt install poppler-utils

On RHEL/CentOS:

sudo yum install poppler-utils

Or compile from source if needed.

pdfunite Merge Syntax

The basic syntax for pdfunite is:

pdfunite input1.pdf input2.pdf ... output.pdf

So poppler uses a similar command structure to pdftk, with the input PDFs followed by the desired output filename.

Let‘s reuse our example PDFs and merge them with pdfunite.

Merging PDF Example with pdfunite

Using the same report-pt1.pdf and report-pt2.pdf input files, here is the poppler command to merge them:

pdfunite report-pt1.pdf report-pt2.pdf full-report.pdf

The output merged PDF is created as full-report.pdf, identically to our pdftk example.

pdfunite provides simple, straightforward PDF merging. But pdftk has a few more handy power user features.

pdftk vs poppler: Feature Comparison

Now that we‘ve seen basic usage for merging PDFs with pdftk and poppler, let‘s compare some of their capabilities:

Feature	pdftk	poppler
Merge all PDFs in folder	Yes	No
Encrypt output with password	Yes	No
Compress output file size	Yes	No
Split single PDF into parts	Yes	Yes
Rotate pages	Yes	Yes
Fill out PDF forms	Yes	Dynamic Forms

Merge all PDFs in folder

pdftk can merge all PDFs in a folder by using *.pdf instead of listing each file:

pdftk *.pdf cat output combined.pdf 

Whereas poppler requires specifying each input file name.

Output encryption, compression and permissions

pdftk provides a wealth of output options missing in poppler like encryption with owner/user passwords, compression levels and permitted access privileges.

Splitting and rotating pages

Both tools can equally split and rotate PDF pages. Poppler additionally allows extracting and arranging pages.

Form filling

While pdftk can populate standard PDF form fields, poppler provides specialized pdffill and pdftrn tools for dynamic PDF forms.

In summary, pdftk edges out poppler for most merging use cases with handy power user features. But poppler offers greater overall PDF manipulation abilities when needed.

With this full perspective, let‘s wrap up with some guidelines on when to use each tool.

Conclusion: When to Use pdftk vs poppler

In this deep dive, we compared pdftk and poppler for easily merging PDFs via the Linux command line.

To recap key differences:

• pdftk – Specialized PDF toolkit with streamlined features for merging, encryption, compression and form population.

• poppler – Robust general PDF library and tools enabling broad manipulation capabilities.

In practice:

• For basic PDF merging needs, both tools can suffice.

• If additional output file controls are needed go with pdftk.

• For heavy PDF editing beyond merging, utilize poppler features like page extraction, splitting and interactive form handling.

So in summary:

• pdftk = Quick PDF merges

• poppler = Advanced PDF editing

Hopefully this guide has provided a comprehensive view into your PDF merging options using the Linux command line. Both pdftk and poppler are powerful allies for wrangling PDF files into shape!