Sentences are usually separated by a period and either one or two spaces. If you have two spaces, you can delete one of them quite easily to reduce the space between those sentences. I would recommend you retain the period and one space.

As you have probably worked out, terminology is important. Are you asking the same question but substituting the word 'sentence' where you really mean 'line' or 'paragraph'?

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Andrew Lockton
Chrysalis Design, Melbourne Australia