PDF Lab / Original specimens

OBSERVED · REPRODUCIBLE · LIMITED

Why visible Korean text is not always editable

A font that displays existing text may not encode new characters. Compare actual replacements in our embedded TrueType subset and Type0 examples.

Start with the problem

Symptom: You can select or copy text, but native editing is blocked or the replacement does not fit.
Main cause: Selectable text is not always replaceable. Editing needs Unicode mappings, supported embedded fonts and an independent text command. NanumGothic substitution can change width and height.
What to do now: Choose Original text · limited, then select a green region or use the right-hand list and read its restriction. Acknowledge font substitution and try a short single line. Shorten text that exceeds the space. Use the originating application if the exact typeface or layout must stay, or no supported region is available.

Did the original responsibility line actually change?

The same first-page region is enlarged at the same scale. Both source and output use the NanumGothic family. This example does not visually demonstrate differences between typefaces.

Before · 가상팀 / Draft 2026. The mixed Korean and English phrase stored in the original PDF. — **Before · 가상팀 / Draft 2026**
The mixed Korean and English phrase stored in the original PDF.

After · 새문팀 / Final 2027. The saved PDF after replacement of the original text command, not a covering text box. — **After · 새문팀 / Final 2027**
The saved PDF after replacement of the original text command, not a covering text box.

Actual PDF crops: same region, 216 dpi. Select an image to open the full PDF.

What changed

“가상팀 / Draft 2026” becomes “새문팀 / Final 2027”. Independent extraction finds the new phrase and no longer finds the old one.

Why it matters

This example embeds the same font family again. The wording also changes, so its different line width cannot be attributed to a typeface change. Other source typefaces may differ in shape and width.

What to do

Check the supported region and font-substitution notice, then try a short single line. Inspect the saved wording, line ending and nearby artwork. Shorten an overlong replacement or edit in the originating application.

Subset example: font names and measured line width

Measurement	Before	After
PDF font name	AAAAAA+NanumGothic	NanumGothic-9742
Extracted phrase bounds	203.094 pt	199.08 pt

Widths use pdfplumber character bounds for this line. Both use the NanumGothic family and the wording changes; this does not isolate a difference between typefaces.

Compare full pages and open the original PDFs

Measured observations

Item	Input / reference	Observed output
Subset-font original	담당: 가상팀 / Draft 2026	담당: 새문팀 / Final 2027
Original with real text	Delivery date: 2026-10-22	Delivery date: 2026-10-23
Type0-font original	담당: 가상팀 / Draft 2026	담당: 새문팀 / Final 2027

These observations apply to these specimens, not every PDF or reader.

When it happens

A PDF may embed only the glyphs it originally used. Replacing “담당: 가상팀 / Draft 2026” with “담당: 새문팀 / Final 2027” needs mappings and glyphs for the new text. Being able to select or copy a phrase does not establish that any replacement can use the same font.

What we did

We authored the phrase in a ReportLab TrueType subset and a pdf-lib Identity-H Type0 document. The actual PDF Desk engine replaces the Tj operand. An independent pypdf check verifies that the old phrase is absent and the new mixed-language phrase extracts correctly. No covering image or whole-page rasterization is involved.

What font substitution changes

Korean replacements embed NanumGothic. Keeping the original point size does not ensure identical glyph shapes or width, so the app requires acknowledgement and checks width and height. Our example uses the same font family; other source fonts may look noticeably different. Overlong replacements are refused instead of squeezed into the original space.

Check it yourself

Download both files and search for 가상팀 and 새문팀.
Open the source in the workbench and choose its original text on the page or in the right-hand list.
Compare the replacement and accept the font substitution before applying it.
Reopen the saved file in another reader, copy the text and compare the chart and unchanged second page.

Fixes and exceptions

Unmapped fonts, TJ arrays, vertical writing, scans, hidden OCR and documents with forms, tags or links are outside this scope. Edit in the source application when selection or fitting fails. Adding a text box is a separate operation: it leaves the original phrase in place.

Files to inspect yourself

Independent observations and method (JSON) · Authorship and file hashes

Crop coordinates, scale, source hashes and fresh extraction · Rechecked: 2026-10-24 · pdftoppm version 26.07.0 · pypdf 6.10.0 · pdfplumber 0.11.9. Original PDFs were not modified.

Environment, date and evidence boundaries

Checked: 2026-10-23 · Windows / Chrome 155 · pdf-lib 1.17.1 · PDF.js 6.4.299 · fontkit 1.1.1 · pypdf 6.10.0 · pdftoppm version 26.07.0. PDFs and artwork are original; NanumGothic uses OFL-1.1. Tables and files record direct observations; explanations interpret those structures. Physical OS keyboard input, phones and every PDF reader are outside this test.

Reproduction code: scripts/lab-inputs.py, lab-outputs.mjs, lab-derived.py, tests/readiness-browser.mjs and tests/lab-pdf.py.

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