Two ways to edit a PDF,
and one of them wrecks it

The moment you notice something is wrong

You open a PDF in an online editor, add a date, save it, and reopen the result. The text looks slightly different. You try to select a paragraph and the cursor will not catch it. Zoom in and the letters are soft. Nothing you did should have caused any of that, and yet the document has clearly been through something.

What actually happened

The editor rebuilt your page. To let you click into existing text and retype it, an editor has to interpret the page's drawing instructions, work out where every glyph sits, let you change them, and then write a new page from that model. Every stage of that is lossy. Fonts that were embedded may be substituted for something approximate. Text positioned by absolute coordinates gets re-flowed. Some editors go further and rasterise the page to an image, at which point the text is not text at all — it is a picture of text, which is why selection stops working and searching finds nothing.

The other approach

A PDF page is a list of drawing operations, and a page's content can be several streams read in order. That means you can append a new stream after the original ones. The original instructions execute exactly as before; yours execute afterwards and draw on top. Nothing is interpreted, nothing is re-flowed, nothing is re-encoded. The page you get back is the page you put in, plus your additions.

What that trades away

You cannot retype the existing text. Adding a layer means adding, so correcting a typo in the original paragraph is not possible — you can only cover it and write over it, which is rarely what you want. If you genuinely need to change body text, go back to the source document and export a new PDF. For the things people actually do to PDFs most often — signing, dating, initialling, stamping, adding a note — the layer approach gives a better result and cannot damage the document.

The bit worth being careful about

Because you are drawing on top, you can put a black rectangle over a name and the page will look redacted. It is not. The text underneath is untouched, and it is still selectable and copyable by anyone who receives the file. This has produced real disclosures in filed court documents and published reports, and it keeps happening precisely because the result looks correct on screen. Covering is not removing. If something must genuinely go, remove it in the source document and export again, or flatten the file by printing it to a new PDF.

Two things to check on any editor

First, open the result and try to select the original text. If you cannot, the page was rasterised and you have lost searchability, accessibility and file size. Second, open the network tab while you use it. Editing a PDF is one of the few operations a browser can do entirely on its own, and a tool that uploads your signed contract to a server in order to add a date is asking for trust it does not need.