This PDF editor adds marks to a file you already have. It is the job people mean by edit PDF online when the task is a name on a form, a highlight on a clause, a scribble in a margin, or a white box over a draft line. It is not the job of rewriting a paragraph so the lines reflow. The original text operators stay where the author put them. Your additions are drawn on top, in a new PDF that you download. The file you picked is not modified on disk until you choose to replace it yourself.
Open a PDF. The page is drawn with pdf.js, the same viewer the compress PDF tool uses, including the standard fonts a text page needs in order to paint letters. Choose a tool, then work on the preview. Text is a line you type and a size; Add text drops it at the last place you clicked, or near the top left if you have not clicked yet. Draw follows the pointer. Highlight and white-out are rectangles you drag. Image places a PNG or JPEG you picked. The signature pad is a small canvas; Place signature puts that drawing on the page as a picture. Undo removes the last mark. Previous and next move between pages, up to 20. Save runs pdf-lib, which is also what merge PDF and JPG to PDF use, and embeds the marks into a new document.
The font on save is Liberation Sans, shipped with this site, so the new words are real text in the PDF rather than a screenshot of the whole page. That font covers Latin letters, which is enough for English and for a lot of Malay, and it is not a Chinese, Japanese, or Arabic face. If a character is outside that range, saving stops and the note says so. Put those words in an image and place the image. A scanned page is already a picture; drawing on it still works, because ink and rectangles do not need a font. White-out is white paint. It hides the pixels or the text in the new file for a casual reader. It is not a redaction tool. Someone with a PDF debugger may still find the old operators under an opaque rectangle. Do not use it for a secret you cannot afford to leak.
Highlights are yellow and partly see-through, so the line under them remains readable. Drawings are a string of short lines in a dark orange. Images keep the proportions you get from a default box, not a cropper; if you need a different shape, crop the picture first in another tool and then place it. The image compressor can shrink a photo before you stamp it onto a page, which matters because a huge photo makes a huge PDF. After you export pages to pictures with PDF to JPG, this editor is the wrong direction: it will not turn those pictures back into live text. Nothing here is OCR.
Limits are there so a tab does not die. Twenty pages, 25 MB, and a pen stroke that forgets its oldest points if you draw for a very long time. Passworded files are not unlocked; this is not an unlocker. A file that pdf.js cannot open will say so in the note instead of pretending. Save can take a moment on a phone because the new file is built in memory. When the download link appears, the new PDF is a normal file. Close the tab and the only copy left is the one you saved. There is no expiry link and no inbox.
Nothing is uploaded. The libraries are loaded after you use the page, from this site, not from a file host that would see the document. There is no account. A contract, a medical letter, or an identity scan is a poor thing to hand to a server you do not run, which is why the work stays here. If you only needed to join two files, use merge PDF. If you only needed pictures on blank pages, use JPG to PDF. This page is the markup step between those. Check the download before you send it. A mark in the wrong place is still your mark.