# QPDF JSON

## Overview

Beginning with qpdf version 8.3.0, the qpdf command-line program can produce a JSON representation of the non-content data in a PDF file. It includes a dump in JSON format of all objects in the PDF file excluding the content of streams. This JSON representation makes it very easy to look in detail at the structure of a given PDF file, and it also provides a great way to work with PDF files programmatically from the command-line in languages that can’t call or link with the qpdf library directly. Note that stream data can be extracted from PDF files using other qpdf command-line options.

## JSON Guarantees

The qpdf JSON representation includes a JSON serialization of the raw objects in the PDF file as well as some computed information in a more easily extracted format. QPDF provides some guarantees about its JSON format. These guarantees are designed to simplify the experience of a developer working with the JSON format.

Compatibility

  The top-level JSON object output is a dictionary. The JSON output contains various nested dictionaries and arrays. With the exception of dictionaries that are populated by the fields of objects from the file, all instances of a dictionary are guaranteed to have exactly the same keys. Future versions of qpdf are free to add additional keys but not to remove keys or change the type of object that a key points to. The qpdf program validates this guarantee, and in the unlikely event that a bug in qpdf should cause it to generate data that doesn’t conform to this rule, it will ask you to file a bug report.

  The top-level JSON structure contains a “`version`” key whose value is simple integer. The value of the `version` key will be incremented if a non-compatible change is made. A non-compatible change would be any change that involves removal of a key, a change to the format of data pointed to by a key, or a semantic change that requires a different interpretation of a previously existing key. A strong effort will be made to avoid breaking compatibility.

Documentation

  The qpdf command can be invoked with the `--json-help` option. This will output a JSON structure that has the same structure as the JSON output that qpdf generates, except that each field in the help output is a description of the corresponding field in the JSON output. The specific guarantees are as follows:

  - A dictionary in the help output means that the corresponding location in the actual JSON output is also a dictionary with exactly the same keys; that is, no keys present in help are absent in the real output, and no keys will be present in the real output that are not in help. As a special case, if the dictionary has a single key whose name starts with `<` and ends with `>`, it means that the JSON output is a dictionary that can have any keys, each of which conforms to the value of the special key. This is used for cases in which the keys of the dictionary are things like object IDs.

  - A string in the help output is a description of the item that appears in the corresponding location of the actual output. The corresponding output can have any format.

  - An array in the help output always contains a single element. It indicates that the corresponding location in the actual output is also an array, and that each element of the array has whatever format is implied by the single element of the help output’s array.

  For example, the help output indicates includes a “`pagelabels`” key whose value is an array of one element. That element is a dictionary with keys “`index`” and “`label`”. In addition to describing the meaning of those keys, this tells you that the actual JSON output will contain a `pagelabels` array, each of whose elements is a dictionary that contains an `index` key, a `label` key, and no other keys.

Directness and Simplicity

  The JSON output contains the value of every object in the file, but it also contains some processed data. This is analogous to how qpdf’s library interface works. The processed data is similar to the helper functions in that it allows you to look at certain aspects of the PDF file without having to understand all the nuances of the PDF specification, while the raw objects allow you to mine the PDF for anything that the higher-level interfaces are lacking.

## Limitations of JSON Representation

There are a few limitations to be aware of with the JSON structure:

- Strings, names, and indirect object references in the original PDF file are all converted to strings in the JSON representation. In the case of a “normal” PDF file, you can tell the difference because a name starts with a slash (`/`), and an indirect object reference looks like `n n R`, but if there were to be a string that looked like a name or indirect object reference, there would be no way to tell this from the JSON output. Note that there are certain cases where you know for sure what something is, such as knowing that dictionary keys in objects are always names and that certain things in the higher-level computed data are known to contain indirect object references.

- The JSON format doesn’t support binary data very well. Mostly the details are not important, but they are presented here for information. When qpdf outputs a string in the JSON representation, it converts the string to UTF-8, assuming usual PDF string semantics. Specifically, if the original string is UTF-16, it is converted to UTF-8. Otherwise, it is assumed to have PDF doc encoding, and is converted to UTF-8 with that assumption. This causes strange things to happen to binary strings. For example, if you had the binary string `<038051>`, this would be output to the JSON as `\u0003•Q` because `03` is not a printable character and `80` is the bullet character in PDF doc encoding and is mapped to the Unicode value `2022`. Since `51` is `Q`, it is output as is. If you wanted to convert back from here to a binary string, would have to recognize Unicode values whose code points are higher than `0xFF` and map those back to their corresponding PDF doc encoding characters. There is no way to tell the difference between a Unicode string that was originally encoded as UTF-16 or one that was converted from PDF doc encoding. In other words, it’s best if you don’t try to use the JSON format to extract binary strings from the PDF file, but if you really had to, it could be done. Note that qpdf’s `--show-object` option does not have this limitation and will reveal the string as encoded in the original file.

## JSON: Special Considerations

For the most part, the built-in JSON help tells you everything you need to know about the JSON format, but there are a few non-obvious things to be aware of:

- While qpdf guarantees that keys present in the help will be present in the output, those fields may be null or empty if the information is not known or absent in the file. Also, if you specify `--json-keys`, the keys that are not listed will be excluded entirely except for those that `--json-help` says are always present.

- In a few places, there are keys with names containing `pageposfrom1`. The values of these keys are null or an integer. If an integer, they point to a page index within the file numbering from 1. Note that JSON indexes from 0, and you would also use 0-based indexing using the API. However, 1-based indexing is easier in this case because the command-line syntax for specifying page ranges is 1-based. If you were going to write a program that looked through the JSON for information about specific pages and then use the command-line to extract those pages, 1-based indexing is easier. Besides, it’s more convenient to subtract 1 from a program in a real programming language than it is to add 1 from shell code.

- The image information included in the `page` section of the JSON output includes the key “`filterable`”. Note that the value of this field may depend on the `--decode-level` that you invoke qpdf with. The JSON output includes a top-level key “`parameters`” that indicates the decode level used for computing whether a stream was filterable. For example, jpeg images will be shown as not filterable by default, but they will be shown as filterable if you run qpdf --json --decode-level=all.