First, congratulations for learning about Adult ADHD and thank you for raising this important question on Medpedia. This phenomenon is more common than most people (including medical experts) would guess.

The connection between certain handwriting difficulties (called "dysgraphia") and untreated ADHD is familiar to top ADHD experts and knowledgeable observers.

Sometimes a child will be diagnosed with dysgraphia by an educational specialist but the ADHD will be missed entirely - and thus a potential treatment for the dysgraphia (as well as primary ADHD symptoms). Such are the gaps in cross-disciplinary knowledge and cooperation among specialties.

To put it simply, neurostimulants such as methylphenidate (generic form of Ritalin) boost the supply of the brain chemical called dopamine, thus improving message transmission to/from/within the brain. In general, this can help with ADHD's core symptoms of impulsivity, distractibility, and inattention, giving the person more control over thoughts and actions.

Here is an in-depth discussion on ADHD-associated writing challenges: http://www.behavioralandbrainfunctions.com/content/2/1/33
Please note this important conclusion:
"These are significant findings as children with motor problems are at risk for learning problems and poorer psychological adjustment. Because motor clumsiness is not a diagnostic criteria in DSM-IV, it is often not assessed when ADHD is diagnosed and the child may go without intervention."

Here are two studies on ADHD and writing difficulties:

1.
http://www.ncbi.nlm.nih.gov/pubmed/16483390
Fine motor skills and effects of methylphenidate in children with attention-deficit-hyperactivity disorder and developmental coordination disorder.
Excerpt:
"Results demonstrated that children with ADHD-DCD performed more poorly on the manual dexterity subtests, had poorer quality of handwriting, and drew more rapidly, more fluently, but less accurately than controls on the graphomotor task. On methylphenidate, manual dexterity and quality of handwriting improved, and strokes on the graphomotor task became less fluent but more accurate. ...Up to 50% of children with ADHD also have motor coordination problems that are severe enough to meet criteria for DCD.”

2.
http://www.ncbi.nlm.nih.gov/pubmed/17710822
ADHD and dysgraphia: underlying mechanisms
Excerpt:
"Multiple complaints in the domain of writing are common among children with Attention Deficit Hyperactivity Disorder (ADHD)... Recommendations for remediation include educational interventions, use of word processing and judicious use of psychostimulants."