Short, honest explanations of the jobs these tools exist to do — including what each one cannot do.
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How to merge several PDFs into one
Open the merge tool and add every file you want in the result. The list order is the output order, so drag the rows until the sequence is right — the number after each filename tells you how many pages it contributes, which makes it easy to spot a file that has slipped into the wrong place. Press merge when you are happy. If the documents came from a scanner that only does one side at a time, merge the two halves with the interleave tool instead, which alternates them page by page.
There are four ways to split, and picking the right one saves a lot of clicking. A page range such as 1-3,5,7-9 extracts exactly the pages you name. A fixed page count produces a new file every N pages, which is what you want when a document has to be broken into equal chunks. One-file-per-page is for turning a scan into individual documents. Odd and even pages exist for double-sided printing workflows. If the PDF has bookmarks, the bookmark splitter is faster than any of them: it turns each chapter into its own file automatically.
Compression re-encodes the images stored inside the document. Drag the quality slider down and watch the projected size; the preview tells you what you are giving up. A scanned document can drop by 80% or more with barely visible damage. A PDF that is mostly text will hardly move, because there is almost nothing to re-encode — in that case the limit is probably your mail provider rather than the file.
Two tools cover almost every case. If the pages are sideways, use rotate and turn them 90° in the direction you need; rotation is applied on top of whatever orientation the page already had, so a page already at 90° plus another 90° becomes 180°. If the whole document came out in reverse — common when a stack of paper went through the scanner the wrong way up — use the reverse tool, which flips the entire sequence in one click.
The page-number tool stamps a running number in the position, font and size you choose. Use {num} for the current page and {total} for the page count, and tick the mirror option if the document will be printed double-sided, so even-page numbers move to the outer edge. The watermark tool handles the other job: a text or image mark, either centred once per page or tiled across it, with adjustable opacity and rotation.
How to turn PDF pages into images, or images into a PDF
Going out: the PDF-to-image tool renders each page at the resolution you pick and downloads them individually or as a ZIP — use it to put a document into a slide deck or a chat message. Going in: the images-to-PDF tool combines JPGs and PNGs into one document, in the order you drag them into. Photographs of a receipt, a whiteboard or a paper form become a single tidy file.
The unlock tool handles both cases, and the distinction matters. A document with an open password needs that password — you type it in, and the tool produces an unprotected copy. A document that opens freely but forbids printing, copying or editing only has permission restrictions, and those are removed without a password. Either way, only do this with documents you have the right to modify; the tool will not help you break into someone else's file.
Drop a square image into the ICO tool, or skip the image entirely and type a letter or emoji instead. Tick the sizes you need — a real favicon should include 16, 32 and 48 pixels so it looks sharp in a browser tab, a bookmark bar and a desktop shortcut. Rounded and circular crops are available, transparent PNGs keep their transparency, and the simulated browser tab at the bottom shows exactly what visitors will see.