Why turn images into a PDF at all
A PDF solves a specific problem a folder of loose images doesn’t: it bundles multiple pages into one file with a guaranteed, fixed page order and a format that renders identically on every device and printer. This matters whenever the destination expects “one document,” not “several attachments” — submitting a multi-page scanned form, turning a set of photographed homework pages into a single assignment upload, archiving a receipt or ID photo series as one shareable file, or preparing a simple photo booklet. Email attachments and upload forms frequently either limit the number of files you can attach or make the recipient open several files in whatever order their file browser happens to sort them — a single PDF avoids both problems entirely.
How the conversion actually works
Each image is decoded to raw pixel data, then embedded as a full-page image inside a new PDF page using a PDF-construction library that assembles the file’s internal structure directly (a PDF is, underneath the reader-facing result, a fairly rigid structured document format with its own object model for pages, embedded resources, and a cross-reference table). Because the images are embedded rather than “printed” through an intermediate rendering step, there’s no unnecessary re-compression of the image data beyond however the source image was already encoded — a PNG stays exactly as sharp, a JPG’s existing compression isn’t stacked with another lossy pass.
Choosing between “fill page” and “fit to standard size”
These two modes serve different purposes. Fill page treats the PDF purely as a delivery container — each page’s dimensions exactly match the image’s own aspect ratio, so there’s no wasted space and nothing looks shrunk or padded. This is the right choice when the PDF’s only job is to hold the images together as one file, such as combining several product photos or a set of receipts. Fit to standard size (A4 or US Letter) is the right choice when the PDF needs to behave like a normal printable document — consistent page size across every page regardless of the source images’ own aspect ratios, sensible margins, and predictable behavior when someone actually prints it. Scanned forms, signed documents, and anything likely to be printed should generally use a standard page size; casual photo collections generally look better filled edge-to-edge.
Ordering and rotating before export
Because a PDF’s page order is fixed once created, getting the sequence right before export matters more than it would for a loose folder of files, where a viewer could always sort by filename or date. The reordering step exists specifically so a multi-page scanned document — cover page, then page 2, then page 3 — ends up in the PDF in the same logical order a reader expects, regardless of what order the files happened to be selected or dropped in. Similarly, correcting a sideways page here (rather than after the PDF is generated) avoids ending up with a finished document where one page is rotated relative to the rest, which is awkward to fix once the PDF already exists without regenerating it entirely.
A note on file size for large batches
Because images are embedded largely as-is, a PDF combining many high-resolution photos can become quite large — a 20-page PDF built from full-resolution phone photos can easily reach tens of megabytes. If the destination has an attachment size limit, running your source images through the Resize or Compress tool first (reducing dimensions or quality to something appropriate for on-screen or standard-print viewing, rather than keeping full camera resolution) before combining them into a PDF will produce a substantially smaller final file with no visible difference for typical viewing or printing purposes.