the PNG specification (), LZWDecode, a filter based on LZW Compression; it can use one of two groups of predictor functions for more compact LZW compression: Predictor 2 from the TIFF 6.0 specification and predictors (filters) from the PNG specification, RunLengthDecode, a simple compression method for streams with repetitive data using the run-length encoding algorithm and the image-specific filters, DCTDecode, a lossy filter based on the JPEG standard, CCITTFaxDecode, a lossless bi-level (black/white) filter based on the Group 3 or Group 4 CCITT (ITU-T) fax compression standard defined in ITU-T T.4 and T.6, JBIG2Decode, a lossy or lossless bi-level (black/white) filter based on the JBIG2 standard, introduced in PDF 1.4, and JPXDecode, a lossy or lossless filter based on the JPEG 2000 standard, introduced in PDF 1.5. Normally all image content in a PDF is embedded in the file. But PDF allows image data to be stored in external files by the use of external streams or Alternate Images. Standardized subsets of PDF, including PDF/A and PDF/X, prohibit these features. Text Text in PDF is represented by text elements in page content streams. A text element specifies that characters should be drawn at certain positions. The characters are specified using the encoding of a selected font resource. A font object in PDF is a description of a digital typeface. It may either describe the characteristics of a typeface, or it may include an embedded font file. The latter case is called an embedded font while the former is called an unembedded font. The font files that may be embedded are based on widely used standard digital font formats: Type 1 (and its compressed variant CFF), TrueType, and (beginning with PDF 1.6) OpenType. Additionally PDF supports the Type 3 variant in which the components of the font are described by PDF graphic operators. Fourteen typefaces, known as the standard 14 fonts, have a special significance in PDF documents: Times (v3) (in regular, italic, bold, and bold italic) Courier (in regular, oblique, bold and bold oblique) Helvetica (v3) (in regular, oblique, bold and bold oblique) Symbol Zapf Dingbats These fonts are sometimes called the base fourteen fonts. These fonts, or suitable substitute fonts with the same metrics, should be available in most PDF readers, but they are not guaranteed to be available in the reader, and may only display correctly if the system has them installed. Fonts may be substituted if they