My PDF mixes A4 and A3 pages; why do the images come out at different sizes?
Because DPI means pixels per inch of page, and an A3 page has more inches than A4. At 150 DPI the A4 pages become 1241 × 1754 px and the A3 page 1754 × 2481 px, and the tool shows you both groups before converting. If you need one size, convert the A3 pages on their own with a smaller zoom factor.
The DPI is fixed; the pages are not
Choosing 150 DPI does not choose an image size; it chooses a pixel density. Each page is multiplied by that density according to its actual dimensions. That makes sense for printing: an A3 page printed from its image comes out at true A3 size with the same sharpness as the A4 pages. It is a surprise when you drop the images into a store gallery or a carousel and one of them towers over the rest.
Read the size lines before converting
PDF to images does not list sizes page by page. It groups them: every page that produces the same size shares one line. A four page file whose third page is A3 shows this at 150 DPI:
Pages 1-2, 4: 1241 × 1754 px.
Pages 3: 1754 × 2481 px.
If you see more than one line and did not expect it, a page in the file has a different size: an appendix, a drawing, or a scan in US Letter. A Letter page gives 1275 × 1651 px at the same DPI, a difference too small to notice by eye in a reader.
Making the images the same size
There are two routes, and the first is cleaner because it never rescales a finished image:
- Convert the A4 pages first: type 1-2 4 in the Pages field, at 150 DPI.
- Then type 3 on its own, switch to Zoom factor and enter 1.473. The A3 page comes out at 1241 × 1754 px, an exact match for the A4 pages.
- The other route: convert everything at one DPI, then shrink the A3 image with the Image resizer to the same width with the ratio locked.
Where does 1.473 come from? An A3 page is about 1.41 times wider than A4, and dividing the 150 DPI zoom, about 2.083, by that gives about 1.473. The tool confirms it gives the matching size, while 1.4733, for example, already makes the height 1755, so watch the size line as you type.
When to leave the sizes alone
For printing or archiving, keep them as they are: every image has the same sharpness per inch, which is what you want. Matching only helps when images sit side by side somewhere that forces one size, and note that a shrunk A3 page shows its details smaller, so fine print on it may need zooming to read.
Common questions
Is the bigger image sharper?
No. All images have the same density. The bigger one is bigger because the page is.
Can the whole file be converted to one pixel size automatically?
No. One DPI or zoom applies to all selected pages. To match sizes, convert each page size as a separate batch.
Do separate batches affect the file names?
No. The name follows the page number in the file, so page 3 keeps its name and its place.