How to edit a PDF online (text boxes, highlights, and markup)

Highlights depend on a usable text layer

A PDF viewer renders the page for display and can overlay positioned text for selection. When you drag across a sentence, the editor uses the selected characters and their coordinates to place highlights, underlines or strikeouts. Complex layouts, unusual encodings and imperfect text positioning can affect that alignment.

Text boxes use page coordinates instead

A text box is anchored to a chosen point rather than existing words. It can therefore be added to many scans and image-only pages. Password restrictions, damaged files, unsupported page structures or editor limits can still prevent editing, so “works on every PDF” would be too broad a promise.

What a scanned PDF needs first

A scan or a photographed page is, to the browser, just a picture; there's no real text underneath to select, so highlighting, underlining, and striking through have nothing to attach to. A markup tool built around real text selection should say so plainly rather than faking a highlight in roughly the right place. The fix is the same one that makes a scan searchable in the first place: running it through OCR first adds a real, invisible, selectable text layer under the image, after which highlighting works exactly the same as on a native PDF.

Keep changes reversible until export

During editing, each mark should remain removable. Exporting creates the finished copy, while the original should be kept separately. Depending on the implementation, marks may become page content or PDF annotations, and that difference affects later editing and viewer behavior.

What to verify in the exported PDF

A sentence spanning two lines can receive separate rectangles for each line. Check the result around columns, rotated pages and tightly spaced text.

Will my highlights and text boxes still be there if I open the file in another PDF viewer? Yes; once saved, they're drawn directly into the page as permanent content, not a separate layer that depends on this tool to display.

A visual highlight normally leaves the underlying text available, but the export still modifies the PDF and can invalidate digital signatures. Highlighting is not redaction: covered text may remain searchable, copyable or recoverable.

Most everyday PDF markup; flagging a line, adding a short note, striking an outdated number; doesn't call for a full editor, just a way to select real text precisely and place a note wherever you click. The one honest limit is a document with no real text to select, and that's a one-step fix, not a dead end.