reduce a jpeg file size

In Inkscape you can import the image, keep it selected, and then open Export Bitmap dialog (Shift + Ctrl + E). There you change the dpi box to a smaller number (the default is 90dpi), try 20 for example. When you change the dpi you will see that the width and height numbers next to it also changed to much smaller numbers. When you're happy with the size just hit export.

Image created this way will be much smaller in size both KB wise and width/height wise. It will probably be what you need if those documents are for viewing on screen, but if you need a high quality print reducing dpi is bad.

Gimp is a raster application that can also help you with that (its also open source). There you can open your .jpeg and choose Save a copy, or Save as and choose a different name for the file. Hit Save a small popup will come up where you can choose the quality option. You can also preview in the image window how quality of the image changes as you move the quality slider. Move it to the left as far as you can so that the documents are still readable. With this method you keep the same width/height of the image.
You can also resize the image using Image > Scale image, it will give a similar result to my first suggestion using Inkscape.

Also using an email account with a greater attachment size capacity can help. I think gmail allows up to 20MB.

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