The 0 mutation PDF engine. Change only what must change.
Under MythosPDF sits the Minimum Mutation Document Engine: a document compiler that treats your PDF as something to preserve, not something to regenerate. One rule governs everything it does: never modify what you do not need to modify.
- ✓Untouched objects are frozen by default
- ✓Geometry is solved by deterministic code, not guessed
- ✓Every output is rerendered and visually verified
Your file stays private. It's kept in this browser so you can come back to it, and our server deletes its working copy automatically.
- 0
- mutations to anything you didn't edit
- 100%
- original fonts, sizes and colours kept
- ∞
- undo steps and saved revisions
- $0
- no sign up, no watermark, no trial
Three steps. 0 mutations.
- Step 1
Understand
Your PDF is parsed into a normalized model of pages, lines, paragraphs, fonts and graphics, with coordinates unified across crop boxes, rotation and transforms.
- Step 2
Solve
Your edit is fitted with exact font metrics. If it needs room, the engine widens its search one ring at a time, moving the smallest possible set of neighbours.
- Step 3
Patch & prove
Only the affected text operators are rewritten. The result is reparsed, rendered and diffed against the original before you ever see it.
Frozen by default
Every object you didn't edit is a hard constraint, not a preference. The engine only relaxes a neighbour when it has proved your edit is impossible without moving it, then moves as little as it can.
Mutation radius search
First it tries to fit your edit in place. Then within its own line, its paragraph, its column, its page. The first radius that works wins, so a one word fix never ripples into a redesign.
Exact typography
Text is shaped with real font engines and measured glyph by glyph. No width is ever estimated from a character count, and fonts are never silently substituted.
Operator level patching
Instead of painting white over old text, the engine rewrites the text showing operators in the page's content stream and preserves the surrounding graphics state: colours, clipping, transforms.
Priorities, not a blended score
Decisions are ranked in a strict order: don't break the document, keep the meaning, keep it readable, then move as little as possible. A small layout win can never buy a fidelity loss.
Visual proof on every export
The same renderer draws the page before and after. If anything outside your edit changed beyond threshold, the export fails loudly instead of shipping a degraded PDF.
Most PDF editors paint over your page. We don't.
A typical online editor hides the old words under a white rectangle and types new ones on top in a fallback font. MythosPDF edits the text itself and leaves every other object alone.
| MythosPDF | Typical overlay editor | |
|---|---|---|
| How text is changed | ✓Rewritten inside the PDF itself | White box pasted over, new text typed on top |
| Fonts | ✓Original embedded font, exact metrics | Often swapped for Arial or Helvetica |
| Longer text | ✓Paragraph rewraps, page reflows | Overflows, overlaps or gets cut off |
| Everything else | ✓Carried over as is (zero mutation) | Page often rerendered or flattened |
| Search & copy | ✓Text stays real, selectable text | Old text often still hidden underneath |
| When quality can't be guaranteed | ✓Tells you, never hands back a broken file | Silently exports whatever it produced |
Questions people ask
+Is "zero mutation" literally zero changes?
The words you edit obviously change, and so does anything that has to move to make room for them. Zero mutation means nothing else changes: no rerendering, no font swaps, no flattening, no nudged images.
+Why not just regenerate the page from HTML?
Because regenerating a page loses what made it yours: kerning, embedded fonts, vector art, exact positions. MythosPDF never converts your PDF into a template; the original file is the source of truth.
+Will my fonts and layout change?
No. Your edit is set in the PDF's own font, at the same size and colour, measured with exact glyph metrics. Nothing is swapped for a lookalike font and nothing else on the page moves unless your edit needs the room.
+What if my new text is longer than the old text?
The paragraph rewraps the way the original was typeset, and only the content below moves down to make room. If the page runs out of space, the overflow continues on a new page inserted right after it.
+Where is my file stored?
A copy stays in your own browser so you can come back and keep editing. Our server keeps a working copy only while you edit and deletes it automatically. We never sell or share documents.
+Can I edit a scanned PDF?
Not yet. A scanned page is a picture of text, not text, so there's nothing to edit. PDFs exported from Word, Google Docs, Canva, payroll or billing software work great.
From the blog
All guides →PDF Reflow Explained: What Happens When Your Edited Text Gets Longer
PDFs don't reflow by themselves. Here's how MythosPDF rewraps paragraphs, moves only the content below, and adds continuation pages without touching the rest.
Can You Edit a Scanned PDF? Text PDFs vs. Image PDFs Explained
Why you can't click to edit text in a scanned PDF, how to tell a scan from a text PDF in 10 seconds, and what your options are.
How to Edit an Invoice PDF: Change Dates, Amounts and Addresses
Fix a wrong date, address, reference or line item on an invoice PDF in under a minute, with totals kept aligned and your billing tool's layout intact.
Your PDF. Your words. Nothing else touched.
Free, in your browser, in under a minute.