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Guides: how PDFs work, and why edits go wrong

Explainers on the parts of a PDF that decide whether a line can be changed: fonts, text objects, saves, signatures, scans and forms.

Whether a line of a PDF can be changed, and whether it still looks right afterwards, is decided by parts of the file most people never see: how the text is stored, which fonts are embedded and how much of each, how the file was saved before, and whether it is signed, scanned, a form or an archive. These guides explain those parts in plain words, starting from what you see on the screen: a font that changes when you edit, letters that will not copy, a file that opens read-only, a size that keeps growing.

Each guide says what causes the problem, how to check for it on a Mac, and what you can do about it, including when the honest answer is that the text cannot be edited where it is. The guides are written from the PDF specification and the documentation of the tools they mention, and each lists its sources with the day they were read. The terms they use have their own entries in the glossary.

Why an edit fails

6 pages

The reasons a line will not change: the file is locked, the page is a scan, the text is a form field, or the text is not stored as letters at all. Each guide starts from what you see on the screen.

The Issues pane of the demo contract: a line that uses a fallback font, and on each page two read-only lines, shared by several pages and taking their font from each page, so they cannot be edited.
The Issues pane lists what the editor found, including the lines it locks and why.

Fonts

5 pages

How fonts live inside a PDF, what happens when one is not embedded, why an edit can change the typeface, and why accented letters go missing.

The full font. Every glyph its designer drew.

The embedded subset. Only the glyphs of “INVOICE DATE”: there is no B, F or G to type.

A full font and the subset a PDF embeds. To keep files small, a PDF often embeds only the glyphs its text uses. A character the document never used is not in the subset, so that font cannot draw it.

What is inside the file

5 pages

How a page stores its text, what hidden text is, why a file keeps growing, and what archival and flattened files mean for an edit.

BT
Begins a text object.
/F1 9.5 Tf
Chooses font F1 at 9.5 points.
380 712 Td
Moves to where the line starts on the page.
(2026-02-28) Tj
Shows the string: each code is a glyph of F1.
ET
Ends the text object.
How a line of text sits in a PDF. In the page’s content stream, a text object chooses a font and a position, then shows a string of codes that the font draws as glyphs.

Privacy, redaction and trust

4 pages

Removing text for good, uploading documents to online editors, telling whether a file was edited, and when editing a PDF is allowed.

Covered. A box hides the line; the words are still in the file.

Redacted. The words are removed and the area is marked.

Edited. The line shows new text in its place.

Covering, redacting and editing are different jobs. Only redaction is meant to remove information. An edit replaces a line on the page, and the same words can remain elsewhere in the file, such as in bookmarks, comments or the file’s properties.

Where to go next

For the steps of a job, such as changing a date or keeping the font, the how-to guides take it from the top. For one term, such as an embedded subset or a ToUnicode map, the glossary has a short entry. For a document you need to correct, the use cases start from the kind of document.