How the converter extracts text from PDF

How the converter extracts text from PDF ? In order to allow all devices to display a unified content format, PDF files use a unique format to record data content, and PDF does not contain text data. The purpose of this article is to let readers, PDF programmers understand the method of extracting text data from PDF files. This article is suitable for those who try to parse binary data in PDF files, but cannot extract text data from it and give up.

Difficulties in extracting text data from PDF

Even if you open a PDF file with a text editor or general programming language, it cannot be used as meaningful data. This is because PDF files are usually binary data and you need to extract the structure by reading the bytes according to the specification. Fortunately, the PDF specification is all published as ISO 32000-1:2008, so writing a program to decipher the binary data in a PDF file is not difficult.

However, just by unraveling the structure of the PDF file, you cannot obtain textual data. Conversely, depending on the PDF file, “characters that make up text data” may not be included in the first place. Instead, the PDF file contains information about which character of which font should be placed where on the screen. This information is sufficient for PDF’s purpose of “reproducing the same appearance in various machine environments”. Text data is not necessary to display PDF files. In short, this is the main reason why extracting text data from PDF files is so difficult.

How the converter extracts text from PDF

Parse binary data to find content stream

First, the binary data is parsed to find the data structure that will become the page when viewing the PDF file. This data structure, called a “content stream”, is scattered throughout the PDF file (as mentioned earlier, this article does not discuss how to find a content stream in a PDF file).

It is confused with “text data”, but in the PDF specification, the characters displayed on the page (that is, the sequence of “characters as pictures”) are simply referred to as “text”. The basic strategy thereafter is to read the text placed on the page from the content stream and interpret it as textual data.

Note that content streams in PDF files are usually compressed. Decompressing it with an appropriate algorithm yields data in plain text. In the following, this data in plain text format is also referred to as “content stream”.

read content stream

Content streams consist of commands called “PDF operators” and their parameters. As you can imagine from the directives and parameters, in order to correctly extract the necessary information from the content stream, it is necessary to write a parser and implement a mechanism equivalent to a stack machine.

In order to assemble the pages to be displayed on the screen, the PDF viewing application also interprets the PDF operators and their parameters to identify “which font and which character should be placed where on the screen”. . A similar mechanism is required for retrieving textual data, as described in the next section. However, you can omit the PDF operators for placing images and PDF operators for managing colors so that you can work more easily.

At least the following four types of PDF operators need to be implemented to extract textual data from a content stream.

4 operators capable of extracting data from PDF files
BT and ET operators to indicate the presence of text in the content stream
Tm and Td operators for positioning text on a page
Tf operator for font selection
TJ operator, Tj operator, etc. for drawing text

AbcdPDF Platform Converter and Online Tools

The above are some ideas shared by people who want to extract file information from PDF. For most users, these technical methods do not need to be considered, because the AbcdPDF platform provides various online tools to allow users to extract PDF file information, merge, Converting to Excel is easy.

pdf can merge multiple PDF files, and the operation is very simple. Through the above technical means, pdf to excel reads the text data of a specific operator from the content stream, and the conversion effect is very good.

It is worth mentioning that word online is a popular online editor for Word, without registration, download and payment, you can edit Word documents online and use rich editing functions.

AbcdPDF Platform Converter


how the converter extracts text from PDF is forever free.this article shows you how to extract information content from PDF files, and three easy-to-use tools on the AbcdPDF platform, namely merge pdf, pdf to excel, and word online, all of which are free forever.

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