Xerox Corp., the world's largest maker of copying machines, reported yesterday that profits slipped 6.6 percent in the final three months of 1981 compared with the same period in 1980, but earnings were up for the year.

Company executives told a securities analysts meeting that unfavorable currency translations, heavy investment in new products and write-offs connected with a major corporate restructuring were responsible for the lower income.

Fourth-quarter earnings fell to $112.7 million ($1.33 a share) from $120.6 million ($1.43) in the last quarter of 1980 as revenue increased 2.4 percent to $2.282 billion from $2.228 billion.

Earnings for the year rose 6 percent to $598.2 million ($7.08) from $564.9 million ($6.69) in 1980. Revenue also rose by 6 percent, to $8.691 billion from $8.197 billion.

Xerox executives said the strengthening U.S. dollar abroad compared with foreign currencies lowered revenues by about 9 percentage points in the fourth quarter and 6 percentage points for 1981.

Delta Airlines, a traditionally profitable carrier, said earnings in its second fiscal quarter ended Dec. 31 plunged 81 percent from a year earlier in the face of the continued slump in air travel.

Net income was $8.54 million (21 cents a share), down from a record $44.66 million ($1.12) a year ago. Revenue rose 4 percent to $894.6 million from $859.2 million.

Six-months earnings totaled $16.15 million (41 cents) compared with $70.98 million ($1.79) for the same period a year earlier. Revenue rose 5 percent to $1.79 billion from $1.70 billion.

PPG Industries reported yesterday that earnings improved slightly in 1981 despite a weak economy and erosion in the company's two major markets.

PPG said it earned $211.2 million ($6.27 a share) on sales of $3.4 billion in 1981. For the previous year, earnings were $209.2 million ($6.34) on sales of $3.2 billion. Despite the increase in sales dollars, the physical volume of goods shipped was slightly lower than in the previous year.

For the last quarter, PPG earned $39.1 million ($1.15), down from $67 million ($2.02) a year earlier. Sales were $803.8 million versus $842 million.

Citing strong results by its magazines, video and forest products operations, Time Inc. said yesterday that its fourth-quarter net income rose 35 1/2 percent to $53.1 million (86 cents a share) from $39.2 million (69 cents) a year earlier. Revenue rose 15 percent to $913.6 million from $795.5 million.

Its 1981 net income was $148.8 million ($2.43) compared with $141.2 million ($2.51) in 1980. Annual revenue rose 18 percent to $3.3 billion from $2.8 billion.

Time said operating income by its magazines unit rose 80 1/2 percent in 1981, while the video operation's profit climbed 32.7 percent and that of its forest products group rose 22.3 percent.

Its books unit's earnings, however, fell 47.6 percent amid the recession at home and abroad, unfavorable exchange rates and other factors, Time said.