Giving an old bitmap a format anyone can open
BMP is the bitmap format Windows has used since its early versions, and it still turns up from Paint, from older office software and from machines that export a picture of a screen or a measurement. A bitmap usually stores every pixel with no compression at all, so one picture can run to many megabytes, and phones, web forms and email services often have no idea what to do with it. A PDF solves both problems at once, provided your browser can read the bitmap to begin with.
Updated
Which tool you need
Bitmaps from Paint, old software or a machine's export
BMP to PDF. Each bitmap becomes one page, with every pixel as it was.
A bitmap refused as soon as you add it
Open it in Paint or another image editor and save a fresh copy. The file uses a form of BMP your browser does not read, and a modern editor writes one it does.
Bitmaps sitting in a folder beside JPG and PNG pictures
Image to PDF takes BMP together with the other common formats, so the folder becomes one PDF in a single pass.
The PDF is smaller than the bitmaps but still too big to send
Compress PDF on the result. Bitmaps holding photographs shrink least on the way in, and that is where compressing helps.
Why the PDF usually shrinks
Most bitmaps are a raw grid of pixels, written out row by row with nothing done to save space. Inside the PDF each one is packed the way a PNG is, without losing anything, and a picture full of flat color or repeated pattern, the usual content of a screen capture or a drawing from Paint, packs very small.
A bitmap holding a photograph shrinks less, because a photo has little repetition to remove, though it still loses no detail. A BMP that the program saving it had already compressed may not shrink by much.
Bitmaps a browser will not read
BMP has gathered variants over the decades: different color depths, a few compression schemes and several header layouts. Your browser opens each bitmap and reads the common forms without trouble; a rare one is turned away by its file name rather than becoming an empty page in the PDF.
Saving it again works because a current editor writes the ordinary form. Nothing is sent to a server for a second attempt, so a bitmap your browser cannot read is one this site cannot read either.
What to know before you start
- BMP files only on this page; a mixed folder needs Image to PDF.
- A bitmap in a variant your browser does not open is refused.
- Words in a bitmap stay part of the picture and cannot be searched.
- Very large bitmaps, or more of them than your device can place, are refused before the PDF is made.