You create a copy of your Raspberry SD card with a command like this:
sudo dd if=/dev/mmcblk0 of=raspbian.dd bs=4M status=progress; sync
Then, if you want to mount that file raspbian.dd in your PC, you can do. First you need to know where it's the offset of the desire partition, it with this commands:
# fdisk -l raspbian.dd
Disk raspbian9.dd: 14.9 GiB, 15931539456 bytes, 31116288 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xf80de80b
Device Boot Start End Sectors Size Id Type
raspbian9.dd1 8192 97889 89698 43.8M c W95 FAT32 (LBA)
raspbian9.dd2 98304 31116287 31017984 14.8G 83 Linux
The important think here is the Boot Start of the partition with the OS (the second .dd2), we have 512 bytes and the partition starts at sector 98304, then:
98304 sectors * 512 bytes = 335544320 SD offset
We have to mount the file .dd with that offset:
# mkdir /media/costales/tmp/
# mount -o offset=335544320 raspbian.dd /media/costales/tmp/
# ls /media/costales/tmp/
bin debootstrap etc lib media opt root sbin sys usr
boot dev home lost+found mnt proc run srv tmp var