Section:

Extracting text from a PDF on iOS

To extract text from a PDF document.

Text extraction reading ordering is not defined in the ISO PDF standard. In fact, there is no concept of sentence, paragraph, tables, or anything similar in a typical PDF file. This means each PDF vendor is left to their own design/solution and will extract text with some differences. Therefore, reading order is not guaranteed to match the order that a typical user reading the document would follow.

The reading order of a magazine, newspaper article, and an academic article are all quite different due to the lack of semantic information in a PDF and the placement/ordering of text in the document. Where different users may have different expectations of the correct reading order.

1PTPDFDoc *doc = [[PTPDFDoc alloc] initWithFilepath: filename];
2PTPage *page = [doc GetPage:1];
3
4PTTextExtractor *txt = [[PTTextExtractor alloc] init];
5[txt Begin: page clip_ptr: 0 flags: 0]; // Read the page.
6
7PTTextExtractorLine *line = [txt GetFirstLine];
8PTWord *word;
9for (; [line IsValid]; line=[line GetNextLine])	{
10    for (word=[line GetFirstWord]; [word IsValid]; word=[word GetNextWord]) {
11        //[word GetString];
12    }
13}

Read a PDF File sample
Full sample code which illustrates the basic text extraction capabilities.

Extract text under an annotation

To extract text from under an annotation in the document.

1PTPDFDoc *doc = [[PTPDFDoc alloc] initWithFilepath: filename];
2PTPage *page = [doc GetPage:1];
3PTAnnot *annotation = [page GetAnnot:0];
4
5PTTextExtractor *txt = [[PTTextExtractor alloc] init];
6[txt Begin: page clip_ptr: 0 flags: 0]; // Read the page.
7NSString* textData = [txt GetTextUnderAnnot: annotation];

About extracting text

When we use the ElementReader class to read elements from a PDF document, we are often faced with data that is partial. For example, let us say that we are attempting to extract a sentence that says "This is a sample sentence." from a PDF document. We could potentially end up with two elements - "T" and "his is a sample sentence.". This is possible because in a PDF document, text objects are not always cleanly organized into words sentences, or paragraphs. The ElementReader class will return Element objects exactly as they are defined in the PDF page content stream.

Text runs

An element of type e_text directly corresponds to a Tj element in the PDF document. Each e_text element represents a text run, which represents a sequence of text glyphs that use the same font and graphics attributes. Say, if there is a single word, whose letters are each presented with a different font, then each letter would be a separate text run. You may also encounter text runs that contain multiple words separated by spaces. The PDF format does not guarantee that the text will be presented in reading order.

TextExtractor class

All this just goes to say that attempting to use an ElementReader to extract text data from a PDF document is not guaranteed to return data in the order expected (reading order). The most straightforward approach to extract words and text from text-runs is using the pdftron.PDF.TextExtractor class, as shown in the TextExtract sample project - TextExtract Sample

TextExtractor will assemble words, lines, and paragraphs, remove duplicate strings, reconstruct text reading order, etc. Using TextExtractor you can also obtain bounding boxes for each word, line, or paragraph (along with style information such as font, color, etc). This information can be used to search for corresponding text elements using ElementReader.

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