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Generating PDF/A From HTML in Meteor


My live-chat app was a folk of project Rocket.Chat which was built with Meteor. The app had a feature that administrative users were able to export the conversations into PDF files. And, they wanted to archive these files for a long time.

I happened to know that PDF/A documents were good for this purpose. It was really frustrated to find a solution with free libraries. Actually, it took me more than two weeks to find a possible approach.

TL, DR;
Using Puppeteer to generate a normal PDF and using PDFBox to load and converting the generated PDF into PDF/A compliance.

What is PDF/A?

Here is a definition from Wikipedia:
PDF/A is an ISO-standardized version of the Portable Document Format (PDF) specialized for use in the archiving and long-term preservation of electronic documents. PDF/A differs from PDF by prohibiting features unsuitable for long-term archiving, such as font linking (as opposed to font embedding) and encryption. The ISO requirements for PDF/A file viewers include color management guidelines, support for embedded fonts, and a user interface for reading embedded annotations.

Why PDF/A documents matter

In my point of view, it’s a standard file for archiving thanks to key features:
  • The fonts of document’s content will always be displayed the same on all places, not depending on any devices.
  • Documents are safe. For example, no executable file launches are allowed or no external content references are allowed.
(Find more features at https://en.wikipedia.org/wiki/PDF/A#Description)

Several approaches for creating a PDF/A document

At the first glance, I found some approaches as follows:

(1). Paid APIs
  • Aspose PDF: Java-based APIs. It seemed the price was too expensive.
  • PdfTron: Javascript-based APIs. I tried to contact them to know the price but they requested me some information even I could not estimate to answer it.
(2). Free APIs
  • Apache FOP: Java-based APIs. I needed to defined the templates by its own ways (FOP files) instead of HTML and CSS. (I created a hello world app here)
  • PDFBox: Java-based APIs. It looked promising that I could convert a normal PDF document to PDF/A one.
(3). Forking a open source project/building my own PDF/A generating engine
  • pdfkit: Javascript-based project. I needed to know the specification of PDF/A compliance, and to build my own lib for generating PDF/A documents.
Basing on these insights, I decided to go with approach (2).

Apache FOP tryout

I decided to stop this approach. Apache FOP strictly required to provide templates with XSL-FO format which is not supported wisely. Ref: https://stackoverflow.com/questions/10641667/use-of-xsl-fo-css3-instead-of-css2-to-create-paginated-documents-like-pdf/21345708#21345708
  • Due to limitation of formatting support, it’s hard to format content of a PDF if its template is complex such as including images, tables, etc..
  • There seemed to be no tools for converting from HTML, CSS into XSL-FO. But my templates were built on HTML, CSS and Meteor and data was bound dynamically.

PDFBox tryout

Basing on the example of creating PDF/A here, I tried to convert an existing normal PDF file into another PDF/A. There were 3 parts to do:
  1. embed/load fonts
  2. include XMP metadata
  3. include color profile
I used these following tools to verify the result:
With filling some mandatory information, I passed the parts 2 and 3 successfully. But, I have really got stuck at part 1 for a long time.

Eventually, I found the root cause by delving deeper into source code of some libraries PDFBox, PDFBox Preflight, node-html-pdf, PhantomJS, etc.. as follows:

I could not successfully generate PDF/A document by converting the existing normal PDFs generated from Meteor because the PDF generated did not embedded the font fully.

My project used node-html-pdf which in turn used PhantomJS to render HTML into PDF. PhantomJS used Qt for writing PDF. The library always embedded the fonts using Subsetting (meaning only the characters were used in the document were embedded). This did not comply with the PDF/A specification.

I couldn’t do anything here because PhantomJS used a very old version of Qt (only Qt v5.x onwards supported PDA/A). The PhantomJS was also discontinued since Mar 2018.

Besides, I could not use PDFBox to embed fonts fully either. There was a method PDType0Font.load for loading fonts but it also embedded the fonts using Subsetting.

Yay! It worked

I discovered that by replacing the process rendering HTML to PDF with the fonts fully embedded.
Using a different library to replace PhantomJS, I found Google Chrome’s Puppeteer.

Then, I could use PDFBox to convert the normal PDF to PDF/A with including XMP metadata and color profile.


References:
[1]. https://www.youtube.com/watch?v=EqII7ilmY8o&feature=youtu.be
[2]. https://stackoverflow.com/questions/38737219/how-to-convert-pdf-to-pdf-a-in-java
[3].http://svn.apache.org/repos/asf/pdfbox/trunk/preflight/src/main/java/org/apache/pdfbox/preflight/PreflightConstants.java
[4]. http://www.pdf-tools.com/public/downloads/whitepapers/whitepaper-pdfa.pdf
[5]. https://medium.com/@raphaelstbler/advanced-pdf-generation-for-node-js-using-puppeteer-e168253e159c

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