I need to prepare A2 file for print, that consists mainly from pictures, that are shot with Canon EOS 40D and is in 10,1MP quality. When placing picture on A2 file, it shows that effective PPI are 202 and resolution 3888 x 2592 px. But I need at least 300 PPI, so I was thinking - when I open RAW file in Photoshop, when saving file, can I resize it from 10MP to 20MP so it goes for 303 effective PPI, and then save as TIFF file? Is that completely normal to prepare images for print on this A2 size?
The required DPI for regular offset printing purposes is derived from the "Halftone Line Screen Frequency" or LPI. The short answer is that the majority of printing can be assumed to be about 150lpi. This used to be considered "high end," where newspapers might be assumed to be 80-100LPI and magazines 100-150LPI.
The trick here is to understand that when the line screen is captured using film or created using digital processes, the images you provide are resampled for 150LPI.
So you don't really need "300dpi," you need 1.5-2 times the line screen (further reading: "nyquist-shannon sampling").
If your plates are at 150LPI for halftone screens, then your images would optimally be 225dpi minimum (150x1.5). Since your effective dpi numbers are pretty close, then I would not worry too much about it. If you are truly concerned, spend 50$ and have your printer pull a proof of a small sample of images at the effective resolution and exact printed size to ease your mind.
Megapixels is merely width x height by the way. Not a good metric.
The best quality possible is always going to be the native capture resolution of the camera without lossy compression. If you resample afterwards either using Photoshop to upsample or merely picking a pixel size larger than the native capture data using a camera RAW tool, you are making up information. If you're only altering size by around maybe .8x to 1.2x then you probably don't even need to think about it. Once you get beyond .5x or 2x then you will notice, even if your audience does not.
Then again, I once accidentally forgot to replace a low-res jpeg stock photo with the proper sized stock photo after a last-minute reworking of a page design. The original was appropriate for 2x2 inches, we went full bleed on 8x8. It looked fine.