A PDF does not contain paragraphs. It contains glyphs with coordinates, and
there is no heading, no list, no table and no guaranteed reading order —
only runs of characters that happen to sit near each other. Converting to
Word means working out where the paragraphs were, from geometry. That
inference is the whole feature, and it is sometimes wrong, so this is called
a conversion and never an edit, and the dialog reports what it found before
anything is written.
Lines are grouped by baseline, runs joined with the spaces a PDF only implies
by leaving a gap, and paragraphs ended where the next line sits unusually far
below, is indented, or where the previous one stopped short of the measure.
Headings come from size relative to the body — which is the most common size
on the page, not the average, because a page of 11 pt under a 28 pt title
averages to something that is neither. Bold and italic come from the font's
name, the only place a PDF records them.
What it refuses to fake is as important. A page set in columns is reported,
not silently interleaved. A page with no text says so, and says why: it is an
image of writing, and reading that needs character recognition this editor
does not have. Tables become plain paragraphs rather than an invented grid,
because a wrong table is harder to repair than no table.
The .docx is written here rather than by a vendored builder: a Word file is a
zip of five XML parts, and the subset that can honestly be produced —
paragraphs of styled runs — is about two hundred lines. Vendoring a document
library would have added another megabyte on top of the four pdf.js and
pdf-lib already weigh, to generate markup we would still have to get right.
Entries are stored rather than deflated, which keeps a compressor out of the
add-on; the CRCs are the part that cannot be skipped, since Word calls the
file corrupt rather than naming the part that upset it.
Text replaced in place converts as replaced. Converting would otherwise hand
back the words the user had just edited away.
Checked by taking the output apart — every CRC verified, both XML parts run
through a real parser — and then, because that is still marking my own
homework, by opening the result in the Word editor extension, where mammoth
reads it with none of my code involved.
Until now "editing" a PDF here meant laying things over it. You could put a
word on top of a word, but the document underneath never changed, and the
result read like a sticker because it was one. This adds the thing the word
Edit actually promises: click a line of the document's text, type different
words, and they land where the old ones were, in the old size and the old
colour.
The position and size come from pdf.js's text layer, which has already placed
a span over every run and carries that run's size in unscaled PDF points — so
the size is right whatever the zoom, which reading it off the rendered box
would not be. The colours come from the rendered page, because nothing in the
text API reports them: the background is the average of the most common colour
bucket in the run's box, since type is a minority of the pixels even when it
is dense, and the ink is whatever sits furthest from that background. On the
test fixture it recovers the marker's red exactly.
Two things that look like details and are not. The bucket only chooses WHICH
pixels are background; the colour itself is their average, because rebuilding
it from the bucket index rounds white down to #f8f8f8 and a not-quite-white
patch on a white page is a visible seam. And the cover reaches below the
baseline by a quarter of the font size, because pdf.js sizes its spans to the
em box: cut the cover to the span and every descender in the original line
survives as a little hook under the replacement.
A replacement is a cover plus text, so it is a mark like any other — movable,
resizable, undoable, and rendered on screen from the same numbers the writer
uses, which is what makes the preview trustworthy.
Said plainly in the dialog and again in the save summary: this hides the
original, it does not remove it. The old glyphs are still in the content
stream underneath. Redact is the tool that takes text away, and it says so
too.
Everything the editor put on a page was final. A text stamp could not be
corrected without deleting it and typing it again, nothing could be resized,
and the only way to remove a mark was a Delete key nobody had been told
about — the selection drew a dashed box and offered no action at all. Placing
a stamp also left its tool armed, so the next click stamped a second copy.
Marks are now editable objects. Selecting one gives it grab handles and a
small bar pinned above it: delete and duplicate for anything, and for text an
edit button, a size stepper and bold and italic. Double-clicking text reopens
it for rewriting in place rather than adding a second one. Placing a text
stamp or a signature drops straight back to the select tool with the new mark
live, which is both what people expect and what puts it immediately within
reach of a nudge.
Resizing is one function over every mark type rather than a special case per
kind: a handle drag produces a new bounding box, and the mark is mapped from
its old box into that one. Text scales by font size instead of stretching its
glyphs, signatures keep their aspect on a corner, and lines offer their two
endpoints instead of a box that would let you stretch them in ways you never
aimed at. A whole gesture lands on the undo stack as one step.
Selecting a thin mark used to mean clicking its outline exactly — about one
screen pixel. Each stroked mark now carries an invisible fat copy of itself
purely to catch the pointer.
New marks to go with it: underline and strike-through, which share the
highlight's text-selection geometry and differ only in where the rule sits; a
plain line; and a fill toggle for rectangles and ellipses. Bold and italic
mean three more Helvetica variants embedded at save time, since a PDF treats
them as separate fonts rather than as a style.
Double-click is detected from the pointer stream rather than from a dblclick
listener, because selecting a mark calls preventDefault() on the pointerdown
and that suppresses the compatibility mouse events the browser would have
synthesised the dblclick from.
A PDF that needs a signature, a highlight or a page removed currently sends
the user out to a desktop application or, worse, to a web service that wants
the document uploaded first. Both are poor answers for a browser whose point
is that nothing has to leave the machine. This is a full-tab editor that opens
a PDF, marks it up, fills its forms and saves a new copy, entirely locally.
Two engines, vendored rather than installed, because an add-on ships as a
self-contained folder over the signed update channel and nothing runs a
package manager on the way: pdf.js reads and renders, pdf-lib writes. They
share no state. Everything in between lives in PDF user space — points,
origin bottom-left — which is the one coordinate vocabulary both speak, so a
mark survives zooming, rotating and reordering with no conversion table and
save-time needs to know nothing about how a page happened to be displayed.
The page strip is built from pdf.js's PDFPageView components rather than its
PDFViewer, which renders pages in the file's own order and cannot hide,
reorder or individually rotate one — three of the features here. Text layers
are ours and stay attached for every page, drawn or not, because Theseus's
find bar is Chromium's findInPage over the live DOM and a torn-down text layer
is a page Ctrl+F cannot see. Canvases are virtualised; a letter page at 100%
is 3.4 MB of bitmap.
Redaction is the part worth being careful about. A black box over text hides
nothing — the text stays in the content stream and comes straight out of a
copy-paste — so the editor says so in a modal before the tool can be used,
and on save rebuilds each redacted page as an image, which genuinely removes
it. Pages that were not redacted are untouched. Form widgets and links are
kept, since they were never the leak.
Saving never writes over the original: every save reloads the source bytes and
replays the session onto a fresh copy, so a botched save cannot poison the
next one.
Out of scope for this first version: editing the text that is already in the
document, and writing XFA forms back (pdf-lib cannot, so those are fill-and-
print only, and the editor says so on open).
Local Dev