You have photographed the four pages of a form, or the front and back of an ID card, and the office wants a single PDF. The photos are in your gallery, the laptop is somewhere else, and the deadline is today. Every modern phone can make that PDF without installing anything, and so can the browser on it. Here are the methods, and the details that decide whether the result is clean or embarrassing.
On iPhone
Scan straight into a PDF. If you have not taken the photos yet, open the Files app, go to a folder, tap the three-dot menu and choose Scan Documents. The camera finds the page edges, straightens the page and saves every page you capture into one PDF. The Notes app has the same scanner under the camera button.
Photos you already took. In the Photos app, select the photos in order, tap Share, then Print. In the print preview, spread two fingers on the first page as if zooming in. The preview opens as a PDF, which you can share or save to Files. It is a hidden trick, but it is built in and it works.
On Android
Scan with Google Drive. In the Drive app, tap the + button and choose Scan. Like the iPhone scanner, it detects edges, flattens the page and lets you add several pages into one PDF, which is saved to Drive.
Photos you already took. Many Android galleries can print to PDF: open a photo, choose Print from the menu and pick Save as PDF as the printer. Doing this for several photos at once varies by phone maker, which is where the browser method is easier.
In the browser, on either phone
Open JPG to PDF in your phone's browser, tap the box and choose the photos. They appear as a list you can reorder with the arrows. Pick A4 as the page size so every page comes out the same size, and press the button. The photos never leave your phone: the PDF is built by the browser itself, which matters when the photos are of an ID card or a bank statement.
Avoid these four problems
Sideways pages. Phones often store photos sideways with a note saying which way up they are. Some PDF tools ignore the note, producing a PDF with pages on their side. The tool on this site reads it, but if a page still comes out turned, fix it with Rotate PDF.
Huge files. A PDF made from four full-resolution photos can be 15 MB, too big for many email servers and every upload portal. Run it through Compress PDF, or, if you scan, use the scanner apps above, which save pages at a sensible resolution.
Wrong order. Galleries sort by time taken, so a page you reshot ends up last. Check the order before converting, or fix it afterwards with Organise PDF.
Shadows and angles. A photo taken at an angle in lamp light makes a page that is hard to read however it is converted. Shoot in daylight, from directly above, with the page filling the frame. The phone scanning guide covers this in detail.
A note on iPhone photo formats
iPhones save photos as HEIC, a format many websites and older computers cannot read. When you choose photos for a website, the iPhone normally converts them to JPEG automatically, so you will not notice. If a site rejects your photos, go to Settings, Camera, Formats and choose Most Compatible; new photos will then be saved as JPEG.