You combine three or four phone photos into a PDF, expecting something modest, and the result is 10 or 15 megabytes. It feels like something went wrong. Usually, nothing did — the tool just did exactly what you asked, and what you asked for was more resolution than a PDF page actually needs.
The resolution mismatch
A modern phone camera captures images around 3,000 to 4,000 pixels wide, often as JPEGs that are 3-5MB each on their own. That resolution makes sense for a photo you might crop, zoom into, or print large. It's far more than a PDF page needs, because a PDF page is typically viewed at a fixed physical size — an A4 sheet, say — and even a high-quality print only needs about 150-300 pixels for every inch of that page. Beyond that point, extra pixels are invisible; they just add file weight.
Naive "combine photos into a PDF" tools — including the simple approach most quick scripts use — take the shortcut of embedding each photo at its full original resolution and just scaling it down visually to fit the page. The file still contains every one of those original pixels; only the *display* size changed, not the actual data. That's the entire reason the output balloons: four 4MB photos in, four 4MB photos still sitting inside the PDF, roughly.
What "properly compressed" actually means here
The fix isn't a mysterious compression algorithm — it's simply resizing each photo down to the pixel count the page will actually display before it goes into the PDF, then re-saving it as a reasonably compressed JPEG. A photo that's being placed at, say, 200mm wide on the page needs roughly 1,200 pixels of width at 150 DPI — nowhere near the 3,000+ pixels a phone camera captured. Doing that resize-then-recompress step before embedding, instead of after, is what separates a 12MB PDF from a 1.5MB one built from the exact same photos.
When you actually want the full resolution kept
There are legitimate cases where you don't want this trade-off — archival scans of documents where every detail might matter later, or a PDF that's specifically meant to be printed at large format. For the vast majority of everyday use — sending a signed form, submitting a document scan, emailing a set of photos as one file — the visual difference between full camera resolution and a properly downscaled, page-appropriate resolution is not something anyone will notice at normal reading zoom.
What to do if you already have an oversized PDF
If the bloated PDF already exists — maybe it came from someone else, or an older tool — you don't need to start over. Running it through a dedicated PDF compression step re-renders each page at a sane resolution and rebuilds the file, which fixes the same problem after the fact.