Discussing the article: "Building a PDF Creation Library in MQL5 (Part 2): Why PDFs Break"
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Check out the new article: Building a PDF Creation Library in MQL5 (Part 2): Why PDFs Break.
Learn how a viewer opens a PDF backwards, why xref entries must be exactly 20 bytes, and how a two-character change falsifies multiple offsets. We reproduce three typical failures, compare results in PDFium and pypdf, and extract four strict checks a generator must pass. The included PdfValidate.mqh class for MQL5 applies these checks and returns actionable offsets and object numbers for reliable testing beyond “it opens”.
A PDF file can open perfectly, display every line of text correctly, and still contain structural errors that go completely unnoticed. This is one of the reasons PDF has a reputation for being difficult to work with. The format itself is relatively simple. The bookkeeping is not, and the bookkeeping can fail silently. In Part 1, we built a PDF by hand. It was 592 bytes long, contained five objects, and opened in every viewer we tested. But that small file was fragile: seven of its numbers represented byte positions, and changing a single character of text could make several of them incorrect. The document might still look exactly as expected, even though its internal references no longer matched its actual contents.
This article explores that fragility through a practical experiment. We will deliberately break the same PDF in three different ways and open each modified file in two independent PDF implementations. All three files will be accepted and all three will still yield the correct text. Neither implementation refuses a file, and the only trace of the damage is a set of warnings that one of them prints only when warnings are explicitly switched on. The central lesson is simple: a PDF opening successfully is not evidence that the PDF is structurally correct. Understanding that distinction is essential when building tools that generate or manipulate PDF files.
Author: Sandro Begashvili