File formats
PDF to JPG or JPG to PDF: Which Way Round, and Why the File Grows
One of these two conversions is what you actually want - and it is the one that most people pick the wrong way round. Here is how to tell.
Zahid Sheeraz5 min readTwo tools, two directions, and people mix them up constantly. The confusion is understandable: both move images and documents around. But the direction you pick changes the result from "exactly what I needed" to "a worse version of what I already had".
Here is the short version. PDF to JPG turns a document into pictures. JPG to PDF turns pictures into a document. Everything else follows from that.
The two directions answer different questions
PDF to JPG is what you want when you need pictures of pages:
- Sending someone a page as an image in a chat or an email
- Putting a PDF page into a slide deck or a social post
- Making a preview or thumbnail of a document
- Proofreading a layout when you cannot open the PDF itself
- Getting a readable copy of a document on a device that struggles with PDFs
JPG to PDF is what you want when you need a document made of pictures:
- Turning a photo of a receipt into something you can email and archive
- Sending a set of photos as one file instead of twelve attachments
- Assembling a portfolio, a mood board or a set of reference images
- Building a form or a set of pages from phone photos
- Converting a set of screenshots into a single shareable file
If you are not sure which one you need, ask what the recipient has to do with it. If they need to read it, they need the PDF. If they need to look at it, they need the image.
PDF to JPG: picking the resolution
The only real decision is the resolution, and it is measured in DPI - dots per inch.
| DPI | Use it for | What it costs you |
|---|---|---|
| 96 | Thumbnails, quick previews, on-screen viewing | Small files, softer text |
| 150 | On-screen use, email, sharing | The sensible default |
| 300 | Printing, anything that will be enlarged | Big files, more memory |
The reason text looks crisp at 150 DPI and blurry at 96 is that the page is being re-imaged at a fixed pixel density. At 150 DPI, a letter-size page becomes about 1275 by 1650 pixels - enough for text to be sharp on a normal screen. At 96, that drops to about 816 by 1056, and small text starts to smear.
The PDF to JPG converter renders each page exactly as it appears, so a scan converts faithfully - a page that was a photograph comes out as a photograph of that page. On a long document, 300 DPI adds up in memory, so it is worth checking the page count before you choose it.
JPG to PDF: why the file grows, and why that is correct
This is the part that surprises people. Four phone photos of 3 MB each become a 12 MB PDF, and the first instinct is that something is wrong. Nothing is wrong - a PDF is not a ZIP file.
A ZIP file stores the photos as compressed bytes and gives them back byte-for-byte. A PDF stores rendered page content: for each page it writes the image at a chosen resolution and quality, and it also writes the page description around it. So the size is the sum of the images at the resolution you chose, plus a small amount of page data.
That means two things:
- Bigger output is not the same as worse output. A PDF that is larger than its source photos usually means the photos are being stored at full resolution rather than thrown away. That is the correct behaviour.
- You can deliberately make it smaller by lowering the resolution or the image quality. Just know that you are removing detail.
There is one quality caveat worth knowing: JPG and PNG images are embedded as they are, with no re-compression, so they come out pixel-for-pixel identical. WebP images are converted to PNG first, because PDF does not handle WebP well, so those can differ slightly from the original.
One page per picture, or several per page
The other decision on the JPG to PDF side is the page layout. You can put one image on one page, or fit several images onto a single page.
One image per page is the default for a reason: it preserves the aspect ratio and the resolution, and nothing is scaled down. Use it for anything you might need to read later.
Several per page is a layout choice - two on a landscape page, four on a portrait page. It is useful for contact sheets, for a photo walk, for a set of receipts on one sheet. The trade is resolution: each image is smaller on the page, and if you then print the PDF, the effective DPI drops.
For a single document that has to be read, one image per page is the right answer. For a visual set, several per page is fine.
Order is part of the job
Both tools let you reorder before you commit. For JPG to PDF it is obvious why: the order of the photos is the order of the pages. Move one up and you have a different document.
For PDF to JPG it matters too, because the output is numbered - and if the pages are meant to be a sequence, a shuffled order is worse than no output at all. Set the order once, deliberately, rather than accepting the default.
When to go the other way
Sometimes the right answer is to convert back. A PDF that a colleague cannot open on their phone is often a PDF that was shared as a file when a picture would have done. And a picture of a page is not always a downgrade: for a quick look, a screenshot beats scrolling a document.
The rule that settles most cases: if the recipient needs to read, comment on or print the content, send a PDF. If they need to see it at a glance, send an image.
A short checklist
- Decide the direction by what the recipient has to do with the file.
- Pick 150 DPI for anything on screen; go to 300 only for print.
- Expect a PDF of photos to be bigger than the photos, and do not "fix" it by recompressing unless you meant to.
- Keep the page order deliberate - reorder before you export, not after.
- Check the first page at full size before you convert the other ninety-nine.