take it with a grain of salt. I made this today, to figure out certain quality aspects of pdf's we received from scanning contractors, to see how they handle big and small pages compared to classic A4 ones. totally a proof of concept. nothing refined to see here.

pdfdpi.py

import pdfplumber
import sys
import glob

if len(sys.argv) < 2:
    print("Usage: pdfdpi.py <file1> <file2> ...")
    sys.exit(1)

files = []

for arg in sys.argv[1:]:
    for fname in glob.glob(arg):
        files.append(fname)

for fname in files:
    with pdfplumber.open(fname) as pdf:
        print(f"File: '{fname}' with {len(pdf.pages)} pages")
        for page in pdf.pages:
            pn = page.page_number
            if not page.images:
                print(f"{pn}: does not contain a scanned image")
                continue
            pw = page.width
            ph = page.height
            
            out = f"{pn} {round(pw/72*25.4)}x{round(ph/72*25.4)}mm DPI: "
            l = len(out)
            for idx, i in enumerate(page.images):
                iw = i["width"]
                ih = i["height"]
                (iW, iH) = i["srcsize"]

                iiw = iw / 72
                iih = ih / 72

                dw = iW / iiw
                dh = iH / iih
                pre = out
                if idx > 0:
                    out += ", "
                color = "unknown"
                cspace = str(i["colorspace"][0])
                if "colorspace" in i:
                    if "RGB" in cspace:
                        color = "RGB"
                    elif "CMYK" in cspace:
                        color = "CMYK"
                    elif "Gray" in cspace:
                        color = "BW"
                issquare = True if round(dw) == round(dh) else False
                dpi = round(dw) if issquare else f"{round(dw)}x{round(dh)}"
                out += f"{dpi}"
                if color != "RGB":
                    out += f" {color}"
            print(out)

print("Press any key to exit")
input()