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Chinese version: Zhihu Step 1: File -- Open, open an image. (If it is a one page PDF file, click Import directly.) Step 2: File -- Export as, change the file extenstion to what you want and the destination folder, then click Export. I tried that this function can convert one page pdf to jpg, you can try to convert multiple pages pdf by yourself.

Chinese version: Zhihu You can use "Scale image" function in GIMP to change the size of an image according to pixel or millimeter. Step 1: File -- Open, open an image. Step 2: Image -- Scale Image. (The image is from Internet, if it infringes copyright, leave a comment and I'll delete it.) Step 3: First choose pixel or millimeter, then choose width and height of the image. Notice to cancle the fixed aspect ratio of original image, or the result may be a little diffirent from your expectation. Step 4: File -- Export as, change the file extenstion to what you want and the destination folder, then click Export. I just tried to scale large size to small size, you can try to scale small size to large size by yourself. By the way, you can use Crop Tool to change the size, but if the size of the original image is larger and very close to the size your want, the result may be wrong. Yo...