How to fix a PCB inside this enclosure?

You buy the complementary inner panel/base plate to mount your products on.

It will fit matching the mounting holes, and it will arrive with the correct set of fasteners.

Or you could just make a PCB matching the mounting holes.

images from Hammond MFG.

The small standoffs molded into the bottom of that plastic box can be used with the proper sized metal machine screws. The screws will cut their own thread in the soft plastic. There are several things to consider:

1. Make sure to select the proper diameter machine screw.
2. Make sure to select a machine screw that is not too long. If too long it can bottom out in the hole and strip (see next item) or even make the plastic in the other side have a bump out.
3. Use care to not over torque the machine screws. It is relatively easy to strip out the holes.
4. When re-mounting an item and re-using a mounting hole that was previously threaded use care to try to get the screw thread to reengage the existing threads instead of trying to form new ones.

Figure 1. Relative size of box-top screws and PCB mounting holes.

If those are M3 screws for the lid then it looks like the mounting posts are about 3.5 mm outside diameter and maybe 1.5 mm inside diameter. You're looking for a short self-tapping screw with a 2 mm thread.

Figure 2. A selection of 2 mm self-tapping screws.

If you have a set of drill bits you can use these as a hole gauge. I use a set with 0.5 mm size increments. The screw thread should be probably about 25 to 33% larger than the hole.