I need to export some embed images to a PDF file:
1. How can I keep the high quality?
2. Will the PDF keep the 300dpi (for printing)?

I see you have had no replies, so I will tell you what I did.

I am an Inkscape newbie, I have only had Inkscape for 2 weeks. My first project was to get business cards printed. I first had to re-create a logo. I then created the business cards and sent them to the printer as pdfs. The end result was a very high quality print.

What I did:
Created a page 90mm x 50mm.

Created the business card.

All text was on its own layer. I duplicated this layer. I made the original text layer invisible. On the duplicated layer, I selected all the text and did Path > Object to Path. This takes care of the event that the printer does not have that particular font on their system. The original text is preserved if I need to edit it.

Then File > Save as Copy and selected pdf as the type. In the dialogue that pops up I chose 600dpi for Resolution for rasterization.

This pdf was sent to printer. Everything on the card was pin sharp, including some 8pt text.

Experienced users are welcome to comment. It will also be a learning opportunity for me.

With embedded images, it might help to check the resolution of the bitmaps and set the size of the Inkscape document accordingly.

e.g. 8.5"x11" at 300dpi... 2550x3300

To my knowledge inkscape doesn't give options how to preserve the embedded image's quality.
For example, you want to print an A4 size pdf, thus you set the page borders to 210/297 mm -and not to pixel values-,
and scale each image to it's correct size.
That way, they will appear in the desired size, but possibly have an uneven resolution value after printing.
That is not a problem at all, with current printers capable for 1200 dpi, they will print a 455,327 dpi image nicely.

The rasterisation value refers to the filters being used, if you don't use any, that setting won't have effect.

So it is not the resolution to worry about, but the jpeg compression.
Embedded images will get that and lose some quality.
And, the level of the compression you cannot even set right through the save as panel.

There is a recent topic yet unanswered on that:
PDF Export with exact jpg copies.

I have no experience with it.
There may be some ghostscript-cairo gurus who know how to do that.
http://www.inkscapeforum.com/viewtopic.php?f=5&t=16775#p62083