What this editor does, and what it does not
Think of it as a clear sheet laid over each page. You can write and draw on the sheet, move things around, and when you save, everything on the sheet is drawn permanently into the PDF with pdf-lib. The page underneath is shown by Mozilla's pdf.js and is never rewritten. That is why you can add a text box but cannot click into an existing paragraph and retype it: changing text that is already in a PDF means rebuilding its layout, which needs the original document.
| You can | You cannot |
|---|---|
| Add text boxes in Helvetica, Times or Courier, any colour, regular or bold | Edit or reflow text that is already in the PDF |
| Cover text with white-out and type over it | Erase the covered text from the file without flattening the page |
| Highlight, box, circle, underline, point with arrows, draw freehand | Add comments or sticky notes that other readers can reply to |
| Place PNG or JPG images such as a logo or a stamp | Open password-protected or edit-locked PDFs |
| Flatten chosen pages to images so hidden text is really gone | Keep flattened pages searchable (they become pictures) |
How a click on screen becomes a position in the PDF
PDF pages are measured in points, 72 to the inch, from the bottom-left corner, as set out in the PDF specification (ISO 32000-1, section 8.3.2.3). Screens are measured in CSS pixels, 96 to the inch, from the top-left. The editor stores every object in points as you see the page, then converts once when you save.
Scale (pixels per point) = zoom ÷ 100 × 96 ÷ 72
Position in points = pixels from the page's top-left corner ÷ scale
PDF x = crop box left + x, PDF y = crop box top − y (unrotated page)
Worked example. A US Letter page is 612 × 792 points. At 150% zoom the scale is 1.5 × 96 ÷ 72 = 2 pixels per point. You click 300 pixels right of and 200 pixels below the page's corner, so the point is 300 ÷ 2 = 150 points across and 200 ÷ 2 = 100 points down. In the PDF's own coordinates that is x = 150 and y = 792 − 100 = 692.
Pages can also carry a rotation, which the specification says turns the page clockwise "when displayed or printed", in steps of 90 degrees. On a page stored with a 90 degree rotation, the same click maps to x = 100 and y = 150 in the file, because the page's own axes are turned. The editor applies this conversion, plus any crop box offset, to every object, so what you place is where it prints.
White-out is a cover, not a deletion
A white rectangle hides text from the eye but not from software. The text is still in the file, and anyone can select it, copy it, or find it with a search. The US National Security Agency's guide Redacting with Confidence calls covering text with boxes "the most common mistake" in electronic documents. If you are hiding an account number, an address or anything private, tick Flatten this page to an image before saving. The page is redrawn as a 150 dpi picture with your edits baked in, and the original text layer is dropped. Expect the file to grow by a few hundred kilobytes per flattened page.
Keyboard and touch
| Action | Keyboard | Touch |
|---|---|---|
| Move the selected item | Arrow keys (1 pt), Shift + arrow (10 pt) | Drag it |
| Resize | Drag a round handle; hold Shift to keep proportions | Drag a handle |
| Delete | Delete or Backspace | Delete button |
| Undo / redo | Ctrl+Z / Ctrl+Y (Cmd on a Mac) | Undo and Redo buttons |
| Edit a text box | Enter, or double-click it | Edit text button |
| Finish typing | Esc or Ctrl+Enter | Tap outside the box |
| Next / previous page | Page Down / Page Up | Next and Prev buttons |
Only the page you are looking at is drawn, so long documents stay light on memory. Very large scans can still be slow on older phones.
Questions people ask
Can I change the words that are already in my PDF?
No. This editor adds new things on top of the page: text boxes, white-out, highlights, shapes, drawings and images. It does not rewrite or reflow the existing text. To correct a word, cover it with white-out and type the new word in a text box on top. The result looks edited, but the old word is still inside the file unless you flatten that page.
Does white-out remove the text underneath?
Only visually. A white box is drawn over the old text, but the text itself stays in the PDF, so it can still be selected, copied, searched or read by a screen reader. Tick "Flatten this page to an image" before saving to really remove it. Flattening turns the whole page into a picture at 150 dpi, so its text is no longer selectable or searchable and links on it stop working.
Is my PDF uploaded to a server?
No. The file is opened by pdf.js and changed by pdf-lib, both running inside this browser tab. Nothing is sent anywhere, and after the page has loaded you can turn off Wi-Fi and the editor still works.
Which fonts can I use for text boxes?
Helvetica, Times and Courier, each in regular or bold. These are the standard PDF fonts, so they need no embedding and look the same in every PDF reader. They cover English and Western European letters (the Windows-1252 set). Chinese, Arabic, Cyrillic, emoji and similar characters are refused with a message instead of turning into empty boxes.
Will my edits land in the right place on rotated or scanned pages?
Yes. Positions are worked out in the orientation you see on screen and then converted to the page's own coordinates, taking account of the page rotation (0, 90, 180 or 270 degrees) and of crop boxes that do not start at the corner. The unit tests for this tool check those cases against pdf.js on real test PDFs.
Can I edit a password-protected PDF?
Not directly. If a PDF asks for a password, or is locked against changes, this tool cannot write to it. If you are allowed to change it, open it in a PDF reader, print it to "Save as PDF" to make an unlocked copy, and edit that copy.