Converting an image to PDF isn't really a conversion in the sense that converting WebP to JPEG is. A PDF is a document container format, not an image format, so what actually happens is the WebP gets decoded to raw pixel data and that pixel data gets embedded as an object inside a PDF page description. The PDF wraps the image with metadata that tells any PDF reader exactly where on the page to place it, at what size, with what margins. This is why a single WebP becomes a document rather than just a renamed file. The PDF format adds document structure (pages, a table of contents reference, font and color space declarations even when no text is present) on top of the image data, which is part of why a PDF containing one image is usually somewhat larger than the source image itself.
The choice between Fit to image and a fixed page size like A4 or Letter comes down to what you're actually trying to produce. Fit to image creates a page that exactly matches the WebP's pixel dimensions, scaled to a sensible physical size. This is the right choice when the image itself is the content you care about, a photo, a screenshot, a piece of artwork, and you want the PDF to behave like a wrapper around it rather than imposing its own layout. A4 or Letter with margins is the right choice when the output needs to look and behave like a printed document, particularly if it's going to be printed physically or if it needs to sit alongside other standard-format documents in the same folder or submission.
Why merging multiple images into one PDF matters more than it seems
The most common reason people need this tool isn't a single image at all, it's several. A set of WebP product photos that need to become a single catalog page, a sequence of WebP screenshots documenting a process, or a collection of saved web images that need to be submitted as one file rather than several separate attachments. Most upload portals and email systems handle one attachment far more reliably than several, and many explicitly require a single PDF rather than multiple image files for document submissions. Merging into one PDF also preserves order in a way that separate file attachments don't: the page sequence in the PDF is fixed, while a folder of individually attached images can get reordered or misread depending on the email client or portal handling them.
The drag-to-reorder step before generating the PDF is worth taking seriously for exactly this reason. Once the PDF is created, the page order is locked in. If you're assembling a document where sequence matters, like a step-by-step process or a chronological set of images, getting the order right before clicking Create PDF saves you from generating the file twice. If some of your source images are a mix of WebP and other formats, the images to PDF tool handles JPG, PNG, WebP and HEIC together in a single batch without needing to convert formats first.