PDFlib TET (Text Extraction Toolkit) reliably extracts text, images and metadata from PDF documents. TET makes available the text contents of a PDF as Unicode strings, plus detailed glyph and font information as well as the position on the page. Raster images are extracted in common raster formats. TET optionally converts PDF documents to an XML-based format called TETML which contains text and metadata as well as resource information.
TET contains advanced content analysis algorithms for determining word boundaries, grouping text into columns and removing redundant text. Using the integrated pCOS interface you can retrieve arbitrary objects from the PDF, such as metadata, interactive elements, etc.
With PDFlib TET you can:
Implement the PDF indexer for a search engine
Repurpose the text and images in PDFs
Convert the contents of PDFs to other formats
Process PDFs based on their contents, e.g. splitting based on headings (requires PDFlib+PDI in addition to TET)
TET supports all relevant flavors of PDF input:
PDF 1.0 up to PDF 1.7 extension level 8 and PDF 2.0, corresponding to Acrobat 1-XI
Protected PDFs which do not require a password for opening the document
Damaged PDF documents will be repaired
Since text in PDF is usually not encoded in Unicode, PDFlib TET normalizes the text in a PDF document to Unicode:
TET converts all text contents to Unicode. In C and other non-Unicode aware languages the text is returned in the UTF-8 or UTF-16 formats, and as native strings in Unicode-capable programming languages.
Ligatures and other multi-character glyphs are decomposed into a sequence of the corresponding Unicode characters.
Glyphs without appropriate Unicode mappings are identified as such, and are mapped to a configurable replacement character in order to avoid misinterpretation.
TET implements various workarounds for problems with specific document creation packages, such as InDesign and TeX documents or PDFs generated on mainframe systems.
TET includes advanced content analysis algorithms:
Patented algorithm for determining word boundaries which is required to retrieve proper words
Recombine the parts of hyphenated words (dehyphenation)
Remove duplicate instances of text, e.g. shadow and artificially bolded text
Recombine paragraphs in reading order
Correctly order text which is scattered over the page
The page content is analyzed to determine text columns. Tables are detected, including cells which span multiple columns. This improves the ordering of the extracted text. Table rows and the contents of each table cell can be identified.
TET provides precise metrics for the text, such as the position on the page, glyph widths, and text direction. Specific areas on the page can be excluded or included in the text extraction, e.g. to ignore headers and footers or margins.
Images on PDF pages can be extracted as TIFF, JPEG, JPEG 2000 or JBIG2 files. Precise geometric information (position, size, and angles) are reported for each image. Fragmented images will be combined to larger images to facilitate repurposing. Image fidelity is guaranteed since no downsampling or color space conversion occurs. This ensures the highest possible image quality.
The TET library includes the pCOS interface for querying details about a PDF document, such as document info and XMP metadata, font lists, page size, and many more.
TET contains special handling and workarounds for various kinds of PDF where the text cannot be extracted correctly with other products. In addition, it includes various configuration features to improve processing of problem documents:
Unicode mapping can be customized via user-supplied tables for mapping character codes or glyph names to Unicode.
PDFlib FontReporter is an auxiliary tool for analyzing fonts, encodings, and glyphs in PDF. It works as a plugin for Adobe Acrobat. This plugin is freely available for Mac and Windows.
Embedded fonts are analyzed to find additional hints which are useful for Unicode mapping. External font files or system fonts are used to improve text extraction results if a font is not embedded.
TET supports various Unicode postprocessing steps which can be used to improve the extracted text:
Foldings preserve, remove or replace characters, e.g. remove punctuation or characters from irrelevant scripts.
Decompositions replace a character with an equivalent sequence of one or more other characters, e.g. replace narrow, wide or vertical Japanese characters or Latin superscript variants with their respective standard counterparts.
Text can be converted to all four Unicode normalization forms, e.g. emit NFC form to meet the requirements for Web text or a database.
PDF documents may contain text in other places than the page contents. While most applications will deal with the page contents only, in many situations other document domains may be relevant as well. TET extracts the text from all of the following document domains:
predefined and custom document info entries
XMP metadata on document and image level
file attachments and PDF portfolios can be processed recursively
general PDF properties can be queried, such as page count, conformance to standards like PDF/A or PDF/X, etc.
TET supports XMP metadata in several ways:
Using the integrated pCOS interface, XMP metadata for the document, individual pages, images, or other parts of the document can be extracted programmatically.
TETML output contains XMP document and image metadata if present in the PDF.
Images extracted in the TIFF or JPEG formats contain image metadata if present in the PDF.
TET optionally represents the PDF contents in an XML flavor called TETML. It contains a variety of PDF information in a form which can easily be processed with common XML tools. TETML contains the actual text plus optionally font and position information, resource details (fonts, images, colorspaces), and metadata.
TETML is governed by a corresponding XML schema to make sure that TET always creates consistent and reliable XML output. TETML can be processed with XSLT stylesheets, e.g. to apply certain filters or to convert TETML to other formats. Sample XSLT stylesheets for processing TETML are included in the TET distribution.
The following fragment shows TETML output with glyph details:
<Box llx="111.48" lly="636.33" urx="161.14" ury="654.33">
<Glyph font="F1" size="18" x="111.48" y="636.33" width="9.65">P</Glyph>
<Glyph font="F1" size="18" x="121.12" y="636.33" width="11.88">D</Glyph>
<Glyph font="F1" size="18" x="133.00" y="636.33" width="8.33">F</Glyph>
<Glyph font="F1" size="18" x="141.33" y="636.33" width="4.88">l</Glyph>
<Glyph font="F1" size="18" x="146.21" y="636.33" width="4.88">i</Glyph>
<Glyph font="F1" size="18" x="151.08" y="636.33" width="10.06">b</Glyph>
TET connectors provide the necessary glue code to interface TET with other software. The following TET connectors make PDF text extraction functionality available for various software environments:
TET connector for the Lucene Search Engine
TET connector for the Solr Search Server
TET connector for the TIKA toolkit
TET connector for Oracle Text
TET connector for MediaWiki
TET PDF IFilter for Microsoft products is available as a separate product. It extracts text and metadata from PDF documents and makes it available to search and retrieval software on Windows.
The TET Cookbook is a collection of programming examples which demonstrate the use of TET for various text and image extraction tasks. Several Cookbook samples show how to combine the TET and PDFlib+PDI products in order to enhance PDF documents, e.g. add bookmarks or links based on the text on the page.