Every request in this category runs into the same wall: change how a character looks and you change how wide it is, and nothing reflows to accommodate it.
What can and cannot be done
| Change | Possible? | Why |
|---|---|---|
| Underline | Yes, as an annotation | Drawn over the text |
| Strikethrough | Yes, as an annotation | Drawn over the text |
| Highlight | Yes, as an annotation | Drawn over the text |
| Text colour | Usually | Does not change width |
| Bold or italic | Poorly | A different font, wider |
| Font size | Poorly | Changes width; nothing reflows |
| Alignment | No | Each glyph is placed individually |
| Line spacing | No | Lines are at fixed coordinates |
The pattern: anything drawn over the text works. Anything that changes the text itself runs into the fact that a PDF has no paragraphs to re-lay out.
Why bold is a font, not a switch
In a word processor, bold is a property. In a PDF, Helvetica and Helvetica-Bold are two separate embedded fonts with different glyph outlines and different widths.
Making a word bold therefore means swapping to a different font. If the bold face was not embedded — and it usually is not, since subsetting only embeds what the document used — there is nothing to swap to. The editor either refuses, or substitutes something with different metrics, which shifts everything after it on that line.
This is also why bold text sometimes disappears when converting a PDF elsewhere. The converter maps the bold font to a substitute that lacks those glyphs, and characters render as nothing. It looks like data loss and is a font mapping failure.
Underline and strikethrough do work
These are the two that behave, because they are annotations drawn over the text rather than changes to it.
Every reader has both in its markup tools. Select the text, apply, and a line object is added at those coordinates. Nothing about the text changes, so nothing reflows.
The trade-off is the usual annotation one: they can be deleted by anyone, and some readers omit annotations when printing. If a strikethrough is legally meaningful — marking a clause as removed — flatten the document so it becomes part of the page. That is covered in how to annotate a PDF.
Alignment and spacing are not properties
This is the part that surprises people most. A PDF does not store "this paragraph is justified" or "line spacing is 1.5".
It stores each glyph at a coordinate. What looks like a justified paragraph is a set of glyphs whose positions happen to produce even margins, computed by whatever created the file. There is no alignment setting to change, and no spacing value to adjust.
Changing either means recalculating the position of every glyph on every affected line — which is what a word processor does and a PDF editor generally will not attempt. If line spacing genuinely matters, the document has to be rebuilt from the source or from extracted text.
What to do instead
Three routes, in order of how much you are prepared to lose.
- Edit the source and re-export. If the Word or Docs file exists, this takes minutes and loses nothing. Everything else here is a workaround for not having it.
- Use annotations for anything that can be drawn on top. Underline, strikethrough, highlight, a note in the margin, a text box over a covered word.
- Extract, rebuild, re-export. Pull the text out with copy or Image to Text, clean the line breaks with Text Cleaner, rebuild in a word processor and export again. You get full formatting control and lose the original layout entirely.
The third route is the honest answer to "make this whole document 1.5 spaced". It is more work than people hope, and it is the only thing that actually produces the result.
Reading, rather than changing
A large share of these requests are really about legibility — the text is too small or too tightly set to read comfortably.
If that is the goal, do not edit the document. Reflow mode re-lays the text into the window at whatever size you choose, ignoring the original layout entirely. It is free, instant and lossless.
That works wherever there is a real text layer, and not at all on a scan. The full picture, including printing larger without touching the file, is in how to change font size in a PDF.
Frequently asked questions
Why can I not make text bold in a PDF?
Because bold is a separate embedded font rather than a property. Helvetica and Helvetica-Bold have different glyph outlines and different widths, and if the bold face was never embedded there is nothing to swap to. Where it works, the wider face overruns the line.
Why did bold text disappear when converting?
The converter mapped the bold font to a substitute lacking those glyphs, so the characters render as nothing. It looks like content loss and is a font mapping failure — the text is still in the file.
Can I underline or strike through text in a PDF?
Yes, both work reliably, because they are annotations drawn over the text rather than changes to it. The trade-off is that anyone can delete them and some readers omit annotations when printing. Flatten the document if the mark is meaningful.
How do I change line spacing in a PDF?
You cannot, in any practical sense. A PDF stores each glyph at a fixed coordinate rather than a spacing value, so changing it means recalculating every line. Rebuild from the source, or extract the text and rebuild in a word processor.
How do I centre or justify text in a PDF?
Also not possible. Alignment is not stored — what looks like a justified paragraph is glyphs whose positions happen to produce even margins. There is no setting to change, only positions to recalculate.
The text is too small to read comfortably. What should I do?
Do not edit the document — use reflow mode, which re-lays the text into the window at any size you choose and ignores the original layout. It is free and lossless. It needs a real text layer, so it does not work on scans.