This may not be the answer you want to hear…
But if you designed the invitations in a low resolution, there is no way to make a high resolution. Resolution is size and quality. For example… a large high resolution picture will have more dots per inch (DPI) than a large picture made at a low resolution.
So if you, for example (These numbers are purely hypothetical, I’m just pulling them out of my ass) made your resolution in 150 DPI, you’re stuck.
You’d need to remake the invitations in a higher DPI, using higher quality images. It’s best to use very large very high resolution images and shrink them down then it is to try to work with low resolutions and make them large.
You absolutely need a program that is designed for this sort of work. Powerpoint is made to make low resolution presentations so that they compress into small files to take on a flash drive or put on a projection viewer.
You need a program for vector work, for image manipulation or for graphic design. Photoshop is a good example of this, but it is expensive. The Gimp is a good free program, and you can make high quality images out of it.
Once it’s a pdf or jpeg you could also just cut/paste into a Word Doc.
This person brings up a valid point I had forgotten. The type of file you save it as will absolutely and definitely effect the quality of your finished product. PDF files are large, but they are not “quality” files. Again, they’re designed to compress neatly so they are easily transferable.
Jpg files are highly compressed files. You will see a lot of artifacting and image distortion and pixelization when you attempt to print this type of image, or if you expand it beyond its already set size. This is a good format for pictures once you have resized them down to the size you want, but they compress heavily and thus do not print well when enlarged.
Edit: Edit: I just want to note that changing the size of your image will not increase the resolution. No, no, no, no no. This will absolutely not help you in the slightest. You will be taking a [very] poor quality image and making it larger thus increasing it’s screen resoution, but you will not be increasing the quality. Lower quality projects can only be scaled down and preserve their limited quality. You absolutely cannot make it any bigger than it is right at this very moment without damaging it further.