See What happens when I send my document to someone else? Will Word mess up my formatting? and Why you should only press Enter once at the end of each paragraph.

You are apparently sending your documents out with the option checked to update styles upon opening. Then your reader's defaults are being applied to your document, rather than your own. Also, pressing the Enter key twice at the end of each paragraph is traditional on a typewriter, not a word processor. Space between paragraphs is achieved by settings in the paragraph spacing for styles.

If you are using Word more than 6 hours a month you owe it to yourself to learn to use Styles in Word. Within two months you will more than have recovered any time you spend learning them. If you use Word more than that the savings are much more dramatic. See Understanding Styles in Microsoft Word and Yet Another "Use Styles" Verbal Beating!

I recently had occasion to edit a 100-page document that was created without using styles. It was formatted completely with direct formatting. Each page ended with a page break.

Each time it had to be saved, the save took more than 60 seconds, during which time Word was frozen. A similar-sized document formatted using Styles takes less than 3 seconds for me to save. This difference was due entirely to the document being directly formatted - a much higher level of complexity. Each paragraph mark in a directly-formatted paragraph carries with it up to fifty different formatting commands. When formatted using Styles, that paragraph mark will carry with it one command - use this style!

Failure to use styles also makes documents much more prone to corruption.