Someone needs six photographs as one file. Not six attachments — one document, in order, that opens the same way on their computer as on yours.
Why a PDF and not six images
A PDF fixes the things a folder of images leaves loose. Order is part of the document rather than a hope about how the recipient's file browser sorts things. Page size is explicit, so what you see is what prints. And it arrives as a single object, which matters when the person receiving it is filing it rather than admiring it.
It also survives the journey better. Images get recompressed by messaging apps, stripped of metadata, reordered, or silently resized. A PDF tends to arrive as it left.
The details that go wrong
Two problems account for most bad photo-to-PDF conversions. The first is orientation. Photographs carry a tag describing how the camera was held, and tools that read the raw pixels while ignoring the tag produce pages that are sideways or upside down. The picture looks fine in your photo library and wrong in the document, which makes it a confusing bug to report.
The second is fit. A landscape photograph on a portrait page has to be either scaled to fit inside the margins, leaving space above and below, or cropped to fill the page. Both are legitimate; silently doing one when you wanted the other is not. A tool should let you choose, and should never distort the image to avoid the decision.
Doing it on the phone
The photographs are already on your phone, which has more than enough processing power to lay them onto pages. Round-tripping them through a website to accomplish that is a habit left over from when phones could not do it.
Recto composes on device: pick the photos, choose a layout, rotate any page that needs it, and export. The orientation tag is honoured, so a photo taken sideways lands upright. Nothing is uploaded, which is worth caring about given how often the photographs in question are of documents rather than scenery.