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go-pdfkit documentation

The whole of PDF in Go, with CGO_ENABLED=0 and no C anywhere: read a file, write one, rearrange it, draw it, read it back as words and pictures, and edit it with other people. Every library here builds for GOOS=js/wasm, so all of it runs in a browser tab.

Fonts are parsed and shaped with go-opentype and pages are rasterised by go-gfx; nothing outside the Go standard library and our own pure-Go libraries is required.

Measured against 118 863 real files

Every claim on this site is measured against a corpus of real PDFs — arXiv's figures, from Matplotlib and Mathematica and pdfTeX and Ghostscript and Adobe — rather than against files written to pass a test. Each wave of work is checked by hashing the pixels of every page before and after and putting the biggest changes beside what the operating system's own renderer draws. That is what found a stroke that came out at half its colour, a font that took the dots off every i, and a colour transform that turned every plot's paper yellow.

Zero C dependencies

No cgo, no bundled libpoppler/libharfbuzz/libfreetype. Glyph subsetting, font-descriptor metrics and per-glyph advances all come from go-opentype's Font.SubsetTrueType / Font.SubsetCFF — pdfkit keeps no private sfnt re-parse or subsetter of its own. See Text & fonts.

The family

Repo What it is
reader reads and writes the format itself: objects, cross-reference tables and streams, filters, encryption, the page tree, content streams — and a writer that produces the same bytes twice
ops the verbs, and the pdfops command: merge, split, rotate, crop, n-up, watermark, encrypt, and reading a page back
render turns a page into pixels: paths, images, text in every font flavour, functions, shadings and patterns
pdffont what a document says about a font — encodings, widths, and what text a code stands for
extract reads a page back: the text with where it sits, and the pictures with the box each covers
coedit a PDF several people edit at once — the plan is shared, not the file
app a PDF workbench that runs in a browser tab and nowhere else
pdfkit the document builder: pages, vector graphics, and text in embedded subsetted fonts
docs this documentation site (MkDocs Material, versioned with mike)

Two libraries outside the org do the work under all of it: go-opentype/opentype, the sfnt, CFF and Type 1 parser and shaper, and go-gfx/gfx, the rasteriser. go-ruby-prawn is the Ruby-idiomatic writer built on the same font stack.

What it is

  • A PDF 1.7 writer: catalog, page tree, cross-reference table and trailer, written to any io.Writer, with optional FlateDecode stream compression (Options.Compress).
  • Vector graphics: paths (move/line/cubic/rect/close), fill and stroke (nonzero and even-odd), DeviceGray/RGB/CMYK colour, line width/cap/join/ miter/dash, CTM transforms (translate/scale/rotate/skew), clipping, q/Q graphics-state save/restore, and constant-alpha transparency via ExtGState.
  • Text as embedded, subsetted fonts: every font used is written as a Type0 (Identity-H) composite font with a per-glyph /W width array and a /ToUnicode CMap for copy/paste. TrueType glyf outlines embed as a subsetted FontFile2 / CIDFontType2 with a /CIDToGIDMap stream; CFF/ OpenType outlines embed as a charstring-subsetted FontFile3 / CIDFontType0.
  • Images: JPEG embedded directly (DCTDecode, no re-encoding); PNG and any image.Image rasterised as XObjects (FlateDecode) with an /SMask for alpha.
  • Widget bridge: Page.AddWidget and Page.AddWidgetVector "print" a go-widgets/toolkit widget tree onto a page — as a rasterised image, or as PDF vector operators with selectable text. See Widgets.
  • Deterministic output: with the zero Options, a document has no timestamps and a content-derived /ID (a SHA-256 of the body), so identical inputs produce byte-identical PDFs.

What it is not (yet)

  • A CID-keyed CFF or a CFF2 (variable) font cannot be charstring-subsetted by the preserve-numbering path, so it falls back to embedding the whole CFF/CFF2 table. A plain (non-CID-keyed) CFF/OpenType font is charstring-subsetted, same as TrueType glyf.
  • Encryption, tagged PDF / PDF-A, interactive forms and annotations are not yet implemented.

See Scope and limitations for the full picture, and the upstream README for the canonical statement.

Install

go get github.com/go-pdfkit/pdfkit

Where to go next

  • Getting started — the five calls that make a PDF: New → AddPage → SetFont → Text → Write.
  • Graphics — paths, colour, line state, transforms, clipping and transparency.
  • Text & fonts — loading fonts, drawing text, measuring and wrapping, and the shaped-text API for complex scripts.
  • Images — DrawImage, DrawPNG and DrawJPEG.
  • Widgets — printing a go-widgets/toolkit widget tree to a page.
  • API reference — the full public surface with Go examples.

Source lives at github.com/go-pdfkit/pdfkit.