Why is each page image several megabytes when the whole PDF was small?
Because a PDF keeps letters and lines as short drawing instructions, while an image keeps the colour of every pixel on the page. An A4 page at 150 DPI is about 2.18 million pixels and at 300 DPI about 8.70 million, four times as many. Lower the DPI, use JPEG for photo pages, or compress the result.
Drawing instructions versus a map of pixels
When Word or Excel saves a quotation as a PDF, it does not save a picture of the page. It saves something closer to: write this sentence in this font at this position, draw a line from here to there. Twenty pages of those instructions can easily stay under a few hundred kilobytes.
A PNG or JPEG knows nothing about letters or lines. It is a grid of coloured squares, and every square has a stored colour. Everything on the page, even the white margin, takes up room in that grid. So turning a light text page into an image always means going from a short description to a full map.
DPI grows the file by the square
An A4 page at 150 DPI: 1241 × 1754 px, about 2.18 million pixels.
The same page at 300 DPI: 2481 × 3508 px, about 8.70 million pixels.
Doubling the DPI doubled both the width and the height, so the pixel count went up roughly fourfold.
File size on disk does not track pixel count exactly, because formats compress areas that look alike, but the direction is clear: each step up in DPI adds weight faster than you would guess. After converting, PDF to images shows each image's size under its thumbnail and the total in the result line, so you can compare for yourself.
Three ways to lighten the images
- Pick the DPI the destination needs and no more. For phones and WhatsApp, 150 usually does, and a plain text page may be fine at 96, which gives 794 × 1123 px.
- For pages full of photos, choose JPEG instead of PNG and try a quality between 75 and 85. PNG is excellent for text and sharp lines but heavy on photographs.
- If you need the high DPI and the files are still big, run them through the Image compressor after converting.
When not to convert at all
If the goal is to send a ten page quotation that the customer will read on their own device, the PDF itself is lighter and clearer than ten images, and its text can be searched. Converting pays off when you need a page to show directly inside a chat or a post, or when the recipient cannot open PDFs. Otherwise you pay in size for a benefit you do not need.
Common questions
Can the image ever be smaller than the PDF?
Yes, when the PDF is packed with high resolution photos and you render it at a low DPI. What counts is the pixel count of the output compared with what the file holds.
Why is the cover page heavier than a text page at the same DPI?
The cover has colours, gradients and photos. Compression gains a lot from the repeated white space of a text page and little from varied detail.
Will lowering the DPI make the text unreadable?
If you go too low, yes. Look at the smallest number on the page after converting, and if it is fuzzy, go up one step.