Here is the information of images in a single-page pdf file:$ pdfimages -list p1.pdf page   num  type   width height color comp bpc  enc interp  object ID---------------------------------------------------------------------   1     0 image     900  1100  rgb     3   8  image  yes        4  0   1     1 mask      900  1100  -       1   1  image  no         4  0    I extract images from the pdf using pdfimages:$ pdfimages -p p1.pdf ./p1$ lsp1-001-000.ppm  p1-001-001.pbm   p1.pdf The outputp1-001-000.ppm and p1-001-001.pbm correspond  to image and mask shown earlier resp.Then I try to recover the original pdf from the image and mask, using convert and pdftk in two different ways (stamp and background):$ for i in *.{ppm,pbm}; do convert $i $i.pdf; done$ lsp1-001-000.ppm  p1-001-000.ppm.pdf  p1-001-001.pbm  p1-001-001.pbm.pdf  p1.pdf$ pdftk p1-001-000.ppm.pdf stamp p1-001-001.pbm.pdf output p1stamp.pdf$ pdftk p1-001-000.ppm.pdf background p1-001-001.pbm.pdf output p1bkg.pdfp1stamp.pdf is blank (very different from original p1.pdf), while p1bkg.pdf and original p1.pdf looksimilar but still different around the right and low borders. The output of pdfimages shown at beginning has a column named typetypeThe image type. Possible values are: image (an opaque image), mask (a monochrome image mask), smask (a soft-mask image) and stencil (a   monochrome mask image used for painging a color or a pattern). Note:  Tranparency in PDF for images is created by using two separate PDF  objects: one for the image and one for the mask or smask. The  mask/smask belonging to a transparent image always directly follows  image in the listing.Are mask, smask and stencile used as foreground or backgroundin a pdf file? How are they different from watermark? Is watermark always used as background?Another issue is that the two new pdf files p1stamp.pdf andp1bkg.pdf are both 476 KB, much larger than the original p1.pdfwhich is only 94 KB. Even if I compress the new pdf files byfollowing command, their sizes don't change:$ pdftk p1bkg.pdf output p1bkgcomp.pdf compressHow can we realistically do the opposite of the splitting work by    pdfimages, i.e. recover original pdf file from their images    extracted by pdfimages. so that the appearance and size are the same or close to the original pdf file?Thanks.
Recover pdf from extracted images
pdf;pdftk